Ex boyfriend/baby daddy

So I'm with someone else right now who is not my children's father but lately I've been spending time with my ex (baby Daddy) when I take my daughters to go see him and we've been talking and I really feel like my feeling for him are coming back really strong and like I really miss him and wish i was back with him sometimes but i dont know what to do I feel really bad for my boyfriend but my ex is also the father of all my kids and I can't help but feel something for him still..also I must say the time we didn't talk and had split up he had a girlfriend who is now pregnant with his baby they are not together anymore as far as I know and he didn't want this baby with her but it's happening and she hates me and I don't like her either but I really miss my ex what should I do..