What to do if you start felling for ur ex but u have a bf?

So recently I visited my family and my ex is friends with my brother so he was around a lot. We all hung out together and it was akward at first but then I started to have feeligs for my ex. I know he still likes me because he told me so, the problem is I have a boyfriend. We've gotten pretty serious but lately we've become really routine. I don't know if my feelings for my ex are genuine or if they are just because of my routine relationship. Either way I can't stop thinking bout my ex. What should I do? If I am behaving in a bad way then how do I stop these feeligs I get about my ex?