Have been doing NC for a while now, found out she has a new guy

Hey everybody,
Have been reading so many comments and discussions on this site which have really helped me get on with my life.
I really love this place.

So here's my story:

I'm 25 years old and she is 26.
I started dating my ex over 5 yrs ago now while she actually was in a relationship. I eventually got her as a girlfriend a few months later and until the end of July this year we had been together (4.5 yrs). We've also lived together for 4 yrs. Probably was a bad idea to get together with someone who wasn't available but she said that she was unhappy with him. We've stuck together really well and me being a great guy got along with her parents. I'm not boasting here with claiming to be a nice/great guy but just so that you get the picture. Problems which I have been doing, which I didn't realize 'til afterwards is that I gave her everything she asked for. But she rarely did anything for me.

Just some info:
Despite being a nice guy I have earlier had mood swings and have had other similar problems. Turns out it's because of something called Hyperthyreosis meaning that I have a really high metabolism. When you're hungry you're not easily in a good mood. I just got the diagnos a few weeks ago and am now being treated for it, but that's a different story.

This summer we went over to London together on our first real trip together. Yes a really long time delayed one as we were in our 4th year together. We had done other trips of course, but they were always together with some other mutual friends. But after that she went 3(!) other times with her friend to London over weekends. I am not the jealous type so I suspected nothing and still don't think anything happened.
When leaving on her 1st trip she had left a note where she wrote that I have been so much better towards her and have stopped fighting over trivial issues which had made her so happy. This really made me love her even more and I was so happy. When she comes back her closest friend (the one she went over to London with) breaks up with her boyfriend, who is the worst type of jealous guy there is and who had told her really unrespecting things about women.

A few days later she herself becomes really irritated one time after a party where she wants me to break up with her. I say that I won't do that as I feel that we're having a great time now. Eventually she says that she doesn't think it to be a good idea to be together, but the next day she says that she doesn't want to break up but that she's not sure she has the same feelings for me as she did before.

After her 2nd trip alone to London (3rd trip in total) we have this great party where I treat her really good and really try to make her feel that I've missed here, which she claimed she didn't feel last time. The day after she meets up with a lot of friends over coffee at different occassions, and then goes over to her moms place. She asks me if I want to come over but also that she won't be there for long, so I figure that there is no use. She comes back the next morning in tears and started packing her bag.

When she broke up with me I reacted in ways that apparently about 95% of everyone does and was destroyed. I asked her why she was leaving but she couldn't give me any real reason she said. She also said that I was the best guy she probably would ever meet but that she had no choice. I figured she had simply fallen out of love with me completely. Later she calls and asks if we can still be friends. I tell that that won't be possible and that I probably won't be able to see her until at least half a year (which turns out may be the time it'll take me to get over her)
Normally I hadn't earlier showed any feelings towards anyone at all really until now. Now I spoke with a lot of different friends who all helped me very much.

She was the type of person who didn't like meeting new people and she found negative things about ALL my friends. Including my mom, sister and best friend. Well actually my sister and my best friend she didn't mind. But I was always really open to her friends and thereby they became our mutual friends.

Now why have I not written anything here earlier? Well I have been doing just fine going NC but just found out she was dating this other guy since around the beginning of October (around my b-day where she wished me a happy b-day and wondered if it was ok. Me being an idiot answers and says that it of course is ok. Thus ending NC that had been going on for a month).

So I found out now december that she's been going out with a guy for two months. My first reaction was one of the worst feelings ever. Same feeling I had when she in 2007 told me she had been unfaithful with a guy one time. I forgave her then. I've been a real whimp when it came to her.

About that:
About a month after the breakup and me becoming the person I wanted to be, I met up with her over coffee. I wanted to show her my new self, but as soon as she came I turned into the same person as I was before. The whimp.

So you can say that I feel SO much better now that I'm a new person, a person that I've always wanted to be with a **** load of confidence. With confidence to flirt with all girls. So this is the person I want to be.
Yet I've really felt that I still want her, but at the same time I REALLY don't.

The feeling I have now, a mixed feeling of which I'm not sure of, is that I'm kind of happy she has a guy. It's as if it has worked as some kind of closure. Which I am happy about. At the same time that I don't want to see her anymore, I also do. I keep in touch with our mutual friends (except for her best friends, mother and step dad) but I always think about her when talking to them.

What do you recommend I do about it? Do I befriend her in order to stop thinking about her and wonder what she's doing or is that a really bad idea?
She's a good person with a lot of good values, but she's messed up psychologically in some points. (Threatened to kill herself holding a knife to her throat back in 2007 if she didn't get any anti-depressants).

Any help is very very welcome.

Wow just writing that made me feel better. First time I did it.
I suppose what I'm also partially asking is how do I get her completely out of my life. Any tips?

/JoeSwede