I have to tell you I have been in your shoes, and I can truly saw that THE FUTURE for you two IS NOT PRETTY!!!
You need to man up and BACK DOWN!!! For many reasons, I'm sorry to say this about women (cause I know there are plenty here that will tell me I'm wrong), but I have enough bad experiences to know what will happen after truamatic periods in life like your situation will cause so many problems in the end.
I'm writing this as a friend to friend, even though we don't even know each other, but because of your current situation, and me going through this with my own ex, you are a friend to me and I need to advise you my friend before you get hurt even more than what your hurting now through time.
Ok, first of all, this pain that your going through will not go away EVER!!! you will no doubt repress and forget it IF and once you get back with her and start a normal relationship with her. BUT it will still be there, this is an emotional pain of hurt betrayal. You will always hold it against her, no matter how much you say you forgive her for everything that she's done with him. You are and she also is stained by this pain, and it will hide inside of you, for the purpose of leaving a happy life together, but it will haunt you constantly, it will surface occasionaly and will cause multiple fights and disagreements over your time together. You will always hold this resentment and will want some kind of payment for making you go through this, you will expect her to be more than a simple GF, because you will hold her to a higher standard that she needs to be yours and will have to be loyal 100%. You will be very uneasy with each other with TRUST, you may say "I trust you" but its just words, you will have an even higher level of jelousy and anxiety for her interactions with other men. She will start hating you for your hightened jelousy and maybe angry episodes that you might make because of all the bad memories. You may try to look deep in your heart to completely forgive her and look even deeper in your mind to release your resentments that have been created against her. But as humans, we act on emotion and what we sense will hurt us, unless we act on what we have experienced in the past. So it will be useless, the future will not be a pretty one, even if it last many years or forever, the pain will build up and knew issues will be created with more pain and more bad memories with more resentment agianst each other which will poison a relationship silently from within. But this is not just the only reason for not going through with this, there are many that I can't even write or know, there are too many things that cause other things to happen. But I'll keep telling you what I do know.
Well, a women will always try to test her limits with you. And because you have already from the beginning set the FOUNDATION of your relationship with you and her and also another guy. With her stuck in between, her choosing who she can be with, (including sexualy) this has given her the POWER to choose, and you have allowed her to think that you are capable of being part of this love game. This has started to promote herself above you, even though it is never spoken and even though she never even thinks it. She now has impressed in her mind the ability of doing if she desires, the process of being able to think the possibility of dating other men. Meaning that she now knows she can be with someone else in the future of your relationship with her (even though she doesn't think it) she sees its not impossible. She knows if she were to ever ingage in another relationship, you will be able to "handle it" and that you have made it seem "ok" already once before, she will feel free to do it agian. regardless of how you threaten her or do anything else drastic from her doing so, even making her fear can't keep this possibility open for her. Why because, you have allowed the horrible experience of her being able to be with two men at the same time a REWARD that you will be at the end for her. So if she does do something, you are now kinda like a safetynet, so she will have you to fall on if she does do something with another. Because you already did it once and whats worse, at the beginning of your relationship, where the foundation is made and the mindset of how to treat each other gets its limits set. Now I have to warn you if you do start suspecting her infidelity, you will only make it worse by fighting and argueing, doing this might only be a temporary fix, but it will not last, infact she will get annoyed and will start wanting to get appart from you. Why because from the foundation, she was allowed to BE ABOVE YOU and have the POWER to choose. So you have no power, yes your the man and yes you have the power of a man, but in mind (her mind) your not in power or ever will be. She is in control of her life and will do whatever she wants, you are not in control of her, no not of her mind, especialy not after she was granted power from the beginning. Like I said before, you will fight each other and draw more and more apart because of the resentment that you have stored, and the her will to test her limits to find what she wants and needs. I'm already writing to much, but I guess I want to get my point across.
Another reason to let it go is (and this might seem like a good strategy to actually conquer her heart, avoiding many problems in a future between you too). Women, are a tricky species to figure out, even they don't truly know what they want. They constantly keep changing their thoughts and wants and needs throughout life. So... Let me tell you, if you would lift up the courage to be a bigger man than the other guy, and let him win, You will actually be the winner in the end. No this isn't a stupid inspirational commercial, this is what happens in a girls mind, they want what they cant have and desire what they don't have. So if you were to back down, you actually become more attractive to her in the end than the other guy. Why because she doesn't have you, and you actually become the POWER, which wemon find very ATTRACTIVE. But this will take TIME, could be month or could be years, (and it can also cause her to never come back to you) but it will take time. You need to be brave and understand that you need to step back and retain your power to be a man and not allow your female partner to take power, (which is the role of a man if you want a good healthy relationship). You already did your job by making yourself known to her and her having strong feeling for you, its set she has an emotional attachment to you. But its wrong that she has two emotional attachments and not just you. So what you need to do is allow him to win. Its a hard choice I know, but its what you have to do to put things right. If you do this, it wont be you in the future thats having all the problems that you would be having in the future with her, but him. He will be haunted with all the resentment feeling and bad memories, he will be having the problems with her being out of control. Its he who will be having a hard time and building up a horrible future not you. Now let me remember that she will be attracted to you, because she will be unhappy with him in time. She will not forget you, she will probly be on her mind whatever happens with her and him. If she does decide to leave him, you can then decide if you want to get together with her, but know with you holding the power, and she will be so drawn to you because you are not below her, but he is. And she will want to choose to be with the person who she finds the most attraction to, a women will always go with what she finds attraction too and will be able to LOVE with RESPECT towards the man that she wants taking care of her in her life. Ok, well do this if you wish, but at your own risk, because this is real life and in this life there are hardly ever any happy endings. But you choose what you want, a life of constant battle for you love, or a brave time of suffering to wait for your love.
But in truth I think you should just forget her! This is not a good way to start and happily ever after relationship, you will only be hurting yourself even more and hurting her to the point of her leaving you anyways in the end.
I know that was alot to read, I guess I just had to write it all down. I don't expect you to do anything I tell you, I just want to give you advise and what I know and have been through, Guess with direction I went through, I guess you can tell by the way of telling how bad I think about wemon. I know there grammer and spelling error, sorry about that and thanks for reading.
If I don't try slicing off my legs with a razor blade, I'll never know what it feels like! Does that mean that I should do it, even if I know it "might" be bad for me?
