Ex asked me back after sleeping with my good friend...Did I do the right thing?

Okay so she isnt technically my ex as we never became 'official', but anyhow, here's a bit of the back story...

So I started seeing this girl from work, mainly because apparently she really liked me and her friends kept hinting at me that I should ask her out. She seemed a nice girl so I did and she said yes. This began a 6 month period of seeing each other, on a casual level.

Now I really started to like her, and after around 6 months I asked her what she thought of becoming more serious. At this point she got very defensive and told me that she doesnt like the thought of getting serious and doesnt want a boyfriend at this time. She explained how she got hurt from a previous relationship and that she's scared of getting put through that again. She did say that she liked me but wanted to take things slow.

After this we became quite distant with each other. It seemed she didnt really know what she wanted and I didnt know where I stood with her so things kind of fizzled out. I was pretty down about it as I did really like her, but carried on as normal. Then 3 months after we stopped everything I found out she had started sleeping with my friend, nothing serious, just sex. I was devastated. I couldnt beleive it. It wasnt technically cheating but it felt like it, however I kept my pride and never said a word to either of them about it, regardless of how much it hurt.

Then a full year after things between us stopped, and only 6 weeks after she last slept with my friend I got a random text off her one night. It was very random as I hadnt even spoke to her for 6 months. It said 'Ok Im just gunna go for it...what would you say to me If I asked if you wanted to make another go of things. I wouldnt blame you if you told me to p**s right off but I just think Im ready for a relationship now and wondered what you thought'. I kept things civil and explained that I really did like her, but I know shes been sleeping with me friend which I really dont like, plus that I was going travelling for 4 months in a week so it wouldnt work anyway. She replied saying 'yeah I dont think id date me neither, wise decision'.

So I went travelling happy in the knowledge that she realised she made the wrong decision and probably regretted it. However when I came back from travelling I found out she had a new boyfriend and annoyingly it bothered me. I realise its very selfish of me to not want her and not want anybody else to have her, but the fact that it bothers me makes me wonder if I did the right thing.

Part of me always thinks what could of been if we made a proper go of things, then part of me thinks the fact she slept with my friend after me and her shows she doesnt really care for my feelings. I would just like to know what other people think of this situation. Did I do the right thing? What would you have done? And how do I move forward?

Thanks for reading :)

Forget about her.