To the ladies who have been cheated on in the past

How did you learn to trust your new partner after being cheated on in a previous relationship? I've been with my boyfriend for 2 years and we were best friends for a year before that. He has never done anything to make him seem untrustworthy. Ever. So I hate that I even question his faithfulness because I know it's because I've been cheated on before and it's not fair to him