Please Help.

I've been in a relationship for 3 years & while my boyfriend is a great father he hasn't been a great boyfriend. He has cheated and lied numerous times and I've always argued but accepted. During the last incident, I made the decision to leave and take our 5 year old child with me (not biologically his child but he has raised him since he was 2.) After him pleading that he would change because he didn't want to lose his family, I decided to come back only if had shown that he really wanted his family. well he has been really working hard to earn his trust back, but ladies it is really hard for me go give it to him. I've gone through alot with him and I love him to death, but sometimes I doubt if a man can really change their ways for a woman once they realize what home is. The damage he has done runs deep and sometimes it's very hard for me to give him even an ounce of trust. Well 2 months into me being back home and he feels like it's been argument after argument because when he tells me something I don't believe him. Am I wrong for not believing him when he tells me something without concrete proof? Am I wrong for being so paranoid? for example: when it takes him a little longer to get home after work. Should I trust him & believe he can change or do I start over?