How can i trust my bf again if he accidentily burned our house down?

Just kidding he didn't burn our house down but i would like some advice on how to trust him again. We've been together for four years since i was 15 and he knew i have been with another guy before we had gotten together and the thought of me with this other guy tore him up constantly he hated me for it although he had been with SIX other girls and after i told him who i was with he refused to tell me who he'd been with but we're past that now. Six months ago I met a girl where i work and we became friends and started hanging out but after four months she told me my bf cheated on me with her two years ago (after we had already been together for two years) my bf refused it but there are some questionable thinks in his and her past so I still don't know who to believe and i have since stopped being friends with this girl. I've found out that three years ago a girl he'd been giving a ride to school had given him a hj and they played around for A WHOLE MONTH and he tried to deny that too. I want to get past it since it was so long ago but I can't stop questioning myself I love this guy so much i've been so good to him and i feel that it would hurt me to have to leave him. I've always been a helpful person so when he's too tired to clean his room i do it or his laundry or his math homework. He's 20 and he's in school but he works full time and his math homework has been suffering so i've taken over and finished a whole semesters worth for him but i love math and i had already done all mine, but i can't help feeling like since im doing something good for him that i'll be finding something bad out and i want to feel goods about myself for helping not nervous. I know i need to tell him how i feel but he refuses to communicate with me he just gets angry and clams up and i feel like im left alone to get over his betrayal and he should be working with me. I need to trust him again for me but how can i get him to help me?