Ex's and friendship

Long story short - went out with a girl shortly after I was officially divorced. I went into it very much as a former nice guy so made an extra effort to change my behaviour. It worked very well and she soon wanted a lot more - I didn't. Eventually its why we broke up. That was back around the end of the new year (briefly tried dating Jan/Feb) but still too much emotion. Since then she has dated a few guys but I haven't seen anyone, we kept in touch now and then because we really clicked with each other. Recently I realised that I was getting to the point where I would like something longer term. We would catch up for coffee, meet up at lunch times and I found myself thinking about her more and more. Now she is dating some other guy and he sounds like quite the catch, when she told me I felt dizzy and nauseous. She still instigates chats and asks to meet up and I'm not sure what to do. I suspect I'm just a back up plan in case things don't work out, in which case I'm tempted to end the friendship before she does. I'm also tempted to end it because I'm not sure if I need the hassle of feeling like some love sick teenager. Then I think maybe I should stick around and step back in at some point because I think she still has feelings for me (she told me she feels sad because we could have made a good team but we cant change the past). She is also really cool to just chat to and maybe would be good for me to treat this as an opportunity to overcome my emotions and not get so emotionally attached to people. Personally I have never believed that friendship is possible between the opposite sex. The fact the sex was AMAZING and I haven't dated anyone since we broke up could also be clouding my judgement ;) At this stage either option seems as good - any advice?

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