Thanks to everyone. Good advice from all. I'm feeling much better. What my psyche has been through is very astounding to me. What everyone was telling me - indeed, what I already knew to be truth - had no effect on how I felt. Quite mysterious. Above all, time is the ultimate healer.
To go into a bit more depth on the awful relationship I had: I knew - I absolutely knew that woman (I'll call her Judy) was using me before I allowed her to move into my home. I knew Judy was a liar. I knew she was a thief. I knew she was an alcoholic. She had me fooled on the drug use, but so what? I accepted the alcoholism, so what's a little meth going to matter? She played the "I'm a little good girl" routine, but I never bought into that. I didn't realize how much of a whore she was, but I never believed (nor would I expect) her to be innocent, especially since she did reveal much of her history which was anything but innocent. Previous to meeting her, it didn't take much for me to tell a woman goodbye. When I met Judy, realizing how messed up she was, I told myself to calm down and give her a chance. Big mistake. At first, I was in complete control of my emotions. Within a month of moving in, I had asked her to leave. She began crying and pleading until I agreed to give her another chance. Second big mistake. A long line of fights, breakups, tenth and eleventh chances ensued, until she had pushed me away so thoroughly that there was nothing left in my heart. She had become a habit (I was probably co-dependent), and it took finding someone else to complete the separation. The lesson I've learned is to never ignore my instincts. I had not ignored them before her, and I haven't since (the woman I met immediately after Judy lasted three dates before I kicked her to the curb, and she was an angel in comparison to Judy). Why I allowed Judy to use me so heavily is beyond my understanding, and I guess I'll just leave it there: as a mystery.
And yes, NOAH_FX. It's never stupid to love someone.