argument help

ok...so this might turn into a long post. but a little background, I have 4 girls from a previous marriage amd am currently in a relationship and am engaged. whenever the girls are gone visiting their dad the first few days without them are nice, my fiance and I are all lovey dovey and enjoying each other company. after about the 3rd or 4th day he brings something up and an argument ensues and we fuss for days. this time he starts off things by asking me if I'm cheating on him. he thinks that because he doesn't feel any romance from me. I try talking to him but end up getting interrupted and eventually end up shutting down. my whole life I have never felt good enough for anyone. I don't have anyone else to talk to please help! I also feel like he can do better than me and want to break it off with him but it hurts so much to even think of leaving him.