In love with my boyfriend but am drawn to my ex?

I've been with my boyfriend for a while and we've been living together since June. I'm very much in love with him and would never want to hurt him. We were on a night out on sat and bumped into my ex. We split very amicably and there was no bad blood or hard feelings whatsoever. We were together for a few months and we liked each other very much but things just got in the way. He'll always be in my life as my mum married his brother and that was one of the things that got in the way, too close to home. Anyway, when we were out on sat, we bumped into him and we were bein friendly enough, nothin ott but I thot he was bein a bit flirty and didn't think it was appropriate given that I was out with my boyfriend. He doesn't like my boyfriend much. He doesn't think he's good for me. My ex has always looked out for me since we split and I know he'd always be there should I need him. Since I seen him on sat, I can't stop thinkin about him and can't help but compare him to my boyfriend and wonder what it would be like if me and my ex were still together. I'm 25 and my boyfriend and ex are both 29. What's goin on??