Feel quite embarrassed speaking about this for all to see, but a problem shared is a problem halved, as they say. 
When I first met my partner, he'd recently just got out of a bad relationship, they'd been broken up 6 months but stopped seeing each other two months before he met me. They'd been together 2 years. We started dating and his ex hung on to him, constantly texting, ringing him when I was with him, begging him to take her back, he said he ignored her or replied with one word answers (which I believe). It caused us a lot of drama as I didn't want this kind of negativity in my new relationship! She knew he was seeing someone yet had no respect for that (even though she'd say she did) she was an absolute snake. He eventually blocked her after 11 weeks. I finally felt relief but knew that wouldn't be the end of it. Lo and behold, a few weeks after he blocked her, she turns up at his door claiming she's moved on and how she would like to be friends (he'd previously told her if she moved on there might be a chance of friendship) i found out about her turning up and hit the roof, he blocked her again, and we started to move on fully.
About a month later I was having a discussion about it all with my niece and after that discussion I continually thought about all the hurt and pain that had been caused and from then on it's been an obsession. I think about it all the time and will bring it up. Our relationship is perfect on so many levels, I trust him but not where it concerns her. I love him to pieces and we get on so so well, he's my best friend and he's said I'm his. He says he wasn't in love with her and the stories he's told me about their relationship are schocking (absolute psycho) I know he's happier with me and he enjoys being with me. I bring a lot to the table and go out my way to make him happy, but I ruin us by bringing it up, I compare myself to her (which I believe is due to the hurt, but dunno) I can't stand anything to do with her, I can't even stand to see/hear her name. I hate how this has affected me as I've never been like this before.
This isnt due to her being an ex, had she been one of those ex's that just checks in every now and again, I wouldn't feel like this. I have no problems with my partners other ex's and feel no ill feeling towards them. It's just her.