Obviously, the situation is complicated. I met this girl seven years ago and we've friends ever since. Back three years ago I started dating a girl I met through her. During that time, the two became best friends, they're really close. The girl I was dating occasionally got somewhat insecure or even a tiny bit jealous and would ask if I ever had any interest in the other girl. Obviously I reassured her and said no, because at the time that was true. We eventually broke up. 

Now today, that is not true. I'm noticing more and more that we have a ton in common, I'm more and more attracted to parts of her personality and I'm becoming more physically attracted to her. I feel as though I'd be betraying not only my ex but now her as well for potentially harming our friendship. This is made even more awkward by the fact that as I said, the two are really close friends. So WIBTA for acting on these feelings? 