I have the same situation... EX of 4 years started talking to me again after 5 months of not seeing each other. This was in March, and I was dating another girl at the time. We talk, had the best sex of my life, and I start to fall for her again. Too much of the past got in the way of a friendship... and it died within 2 months. She started dating another guy right away, and actually got serious.. and I was proud for her. I wanted nothing more than to be happy without me.
Mid July, I heard a song we used to always listen to on a CD. I thought about her for an entire week before I called her. After calling her we catch up that following Monday for a late night acoustic season at a local bar. We did nothing but talk, and be friends, and I had no feelings attached. Next week we kiss, and it goes down hill. She wants to come back to my place and forget about her boyfriend for the night, but I tell her no. The following Monday, we hang out at the bar all night drinking and throwing out starting to date again. She ends up coming back, and hooked up for hours before she left at like 4am.
Now she is starting to feel bad, and when we catch up, she leaves after an hour because she doesn't want to cheat on her boyfriend, but she wants to be back with me in every way possible... even down to a relationship again. It has been on my mind for the past month.... and it sucks. I will never take her back... but the thoughts are still there.
shitty