About 8 months ago I parted ways with a girl I had been with for 3 years. We had both seen it coming, and we both knew that it was the right thing to do. Before we went to uni, everything seemed perfect. While we were at uni, things were changing. She changed quite a bit, while I (as my friend put it the other night) 'mellowed out'. Breaking up sucked more than anything in the world, because neither of us wanted to lose what we had, but we both knew that it was for the best.
I was convinced that I still loved her, but I think that I loved the girl that she used to be. She said pretty much the same stuff to me that your ex said to you - hope we can still be friends, I want the chance to see other guys while I'm still young, blah blah blah... The seeing other guys thing seems to be working out for her - I don't know what's going on, but I know more than I need to.
The friends thing? Well, for a few months after the breakup we got together every 3 or 4 weeks, had coffee, chatted about what was going on in our lives, all that stuff. I last spoke to her properly in October. I saw her at Christmas, and although we were around each other for several hours (we went busking together), we didn't really talk properly about anything. The whole time she seemed like she was annoyed at me, although I couldn't think of anything that I had done to aggravate her.
Then we saw each other again at a New Years party. We barely said 2 words to each other the whole time - it felt like she was ignoring me, and I tried to speak to her a couple of times, but she just gave me short answers and then moved away, and it almost seemed like every time I moved even vaguely in her direction, she found somewhere else to be. I didn't understand what was going on, and when I said goodbye later on she was really cold to me. We haven't seen or spoken to each other since then, and I don't care. Right now I don't care what/who she is doing, and it feels quite liberating.
My advice to you is that you shouldn't work too hard at being friends - don't go out of your way to meet up, don't force conversation. Sure, meet up for coffee or whatever, but don't get hung up about being her friend, because things between you are never going to be the same. You'll make small talk about uni/work/weather/whatever, but she isn't going to talk to you about anything personal at all, and I can't think of any of my good friends who I can hang out with and just talk about uni or work - that is 'acquaintance' stuff - people who you know, but don't REALLY care about.
So get on with your life, say hi to her, but don't worry too much about actually being her friend - if it's possible then it will happen naturally, if its not possible then you will just force things to become awkward.
EDIT: Sorry for the stupidly long post.
Man, what a story, in someways I am glad I'm older.You guys go through a lot of pain. Again I have never been in love.
I would have a hard time dealing with women I have been with knowing they are screwing other guys.