I've fallen for my best friend?

ok I’ve fallen for my best friend… we are pretty close and the other person is in a relationship at the time well has been for a while now .. even before we met…. we do all sorts of things together and have alot of fun together… good laughs and good times, sometimes it seems like we make excuses to meet eachother and it seems like they have an interest in me also, but I don’t know what to do. Maybe its just better to stop hangin out with eachother but I cant seem to get the person off my mind. I know this has probably popped up a million times and I’ve read other posts online about it but thought what the heck why not ask again. It’s just so confusing.