How can someone change so much

My husband was such a sweet guy when I married him and I loved him so much. How do you just let the past go? We were really doing ok until I found out he is still hanging out with that girl even if it is with other people. Everyone knows how much that skank likes him, but even though he is hurting me he is still hanging out with her. My husband never would have done that. Then I call him to tell him that it also bothers me that he goes places with her and hangs out at his buddies house with her. Also, that instead of taking his space and thinking about stuff he is just hanging out with people. That's not fair when I am sitting here waiting for him and I am done. I just can't wait anymore. It's so hard to give up all the love I had for him and pretend the past three and a half years of good times never happened. But I can't handle the person he is know always yelling and cussing at me when I am telling him how I am feelings, since when he decided this he never thought about my feelings. I am having such a hard time giving it up, but I can't cry all the time anymore and I have to get past this because every time I look at the baby and see her looking at the door for her father I cry and I can't keep it up. I love my hhusband, but who ever this guy is is a jerk and I can't pretend anymore that we can go back and I won't go forward with this guy. If the old husband comes back then I would try, but I can't keep trying with this one.