That's awful. It would have been good for her to know the truth.
The problem is really the amount of time. If you were gonna tell her it really needed to be around the time you donated the blood, rather than many years after when she's been happily married for several years.
You could try to be friends with her, but it'll be probably be too hard to not reference your feelings for her if you did.
There are two things you can do:
The first option is to try and think of reasons to not like her or respect her. As hard as that may sound, the only way to become truly detached is to convince yourself that she isn't someone you would want to be with. You have to convince yourself that all the great things you believe are true of her, aren't. Someday, many years in the future, you may be able to believe some of those things are true again, when you're over it and detached. You also need to meditate and try to remove her from your mind. If she's integrated in the way you think then you need to really meditate, restructure your mind and exclude her from it. Imagine a link (ie. a wire) between you and her being severed in your mind whenever you think of her.
The second option is to not bother detaching yourself from her. Instead of detaching yourself from her, you could just think that you WANT to be with her in the event her marriage falls apart, but if you find someone else before that happens, then you're not going to wait for her.
The lesson you learn in the end is to never let yourself attach that deeply to someone until you're married. That you shouldn't take someone for granted and integrate them in the way you think until you're married to them.
You know, the more I try to let her go, the more she comes to my mind.
No pain, no gain.
It's like withdrawing from an addiction - at first it's easy, then it gets harder and harder, but after that you're over it. Although with love that process can take months instead of days.
Romantic music will bother you until you're over it (which could be months or years). My advice is to avoid romantic music as much as possible until you've overcome it, and try not to pay attention to it if you're forced to hear it.
It's best to try and occupy yourself with things that require your undivided attention. This is the best advice that most people give me and that worked for me. It's also a good idea to be talking to people (which hopefully you'll manage here if you have skype?).
It'll be some time before you're ready for a serious relationship, but you could try to find somewhere else to focus your love meanwhile - such as pets.
You can think that you donated blood just because you're a nice guy.