Have taken back my boyfriend that cheated on me. Big mistake???-sorry i kow iv asked before!?

My b/f's relationship with his ex, was about 5 weeks long.I was good friends with his ex, unitl they split and we started a relationship.We have been going out for 2 1/2 yrs.Me and his ex stopped being friends, but still talked now and then.My b/f and her continued being friends after they split.I always felt insecure about her because i knew that she still wanted him, just by seeing the texts she would send him. Recently my friends told me they had kissed twice, on 2 different occasions,no alchol invovled.After a couple days of him begging and crying i took him back.He had already stopped talking to her before i found out. Now he has completely erased her,lets me go through his phones and anything.He seems really sorry, and has begged and sworn on his mum and grandma it will never happen again and that it was the biggest mistake of his life.though the last 9 months i have been meeting my "ex crush" behind his bk, never cheated though, please help!