it doesnt matter what type of relationship that ended, after awhile, the pain has eased and we are left remembering all the good times that were shared, and then we start fantasising about what could have been. this ignites those emotions all over again. but from someone who has done the whole, on again, off again relationship, the bad bits ALWAYS come back.
someone once told me....you should never go BACK to a relationship. you should only move FORWARD towards one.
the way i now let go (this is the final stage of grief btw) is to ask myself, what did i learn about myself in that relationship, what traits in this relationship do i not want in my next, and what ones did i like. you dont have to think bad things about your last gf, nor do you have to regret the relationship. you were meant to meet and go down that track, but if you go back, then it just means that you have not finished learning the lesson you were given.
just my view. hope that helps.