Having trust issues with my soon to be husband?

my fiance has been talking to his ex on and off our whole 9mos relationship. our last break up he had her and her daughter (not his) over his house but claims he didnt sleep with her just played with her kid. every time he ends up changing his num she gets it again or they talk on myspace. he said it was because he wasnt sure that i was serious and these last 2 months since we made the engagedment committment he has not talked to her, or her him. he says she just stopped after he told her off but its happened before and im scared to marry him and get hurt. i also feel like that day they spent together with her child he cheated on me, because he was talking to even before. please some adult advice please.