Okay me and my boyfriend had been going out for 3 months and he says he doesn't trust me and I want to know why if I'm a good girl? At first he told me he does trust me and now he's telling me that he doesn't and I want to know if he really does loves me cause he tells me all the time he does but I don't know if I should believe him cause he cheated in me once but he said sorry and I apologized to him

There's only one person who can answer this question.

If he cheated on you, why did you apologize to him? Maybe he does not trust you because he can't be trusted.
You have been dating 3 months and he has cheated already? Why are you still with him?