What is up with this girl?

Me and my fiancee have been together for 4 and a half years, she has two kids in a previous relationship and i love them as if they were my own. We kind of separated in august because she accused me of cheating. When I tried to cut her off in october she left me a message and was hysterically crying for me to come back and I did. This is a theme whenever I try to cut her off she stops it. I love her and Im sure she loves me but I found out that she has been the one cheating all along, I have officially cut her off but now im having regrets, what should I do please help. The kids adore me and miss me so much thanksgiving was supposed to be our reunitement but she got really depressed. She says she loves me and goes nuts when I dont talk to her but she has been seeing another guy what do i do?