Relationship help please ?

my boyfriend has had a bad past involving girls. Now that he has changed and chosen to move on, its affecting him. On the other hand, its affecting ME because i hear alot of things from people which i trust but then he told me that it wasnt true. Sometimes it makes it hard for me to believe him. The fact that his past is running after him now is really heartbreaking and i have been there for him throughout. But ive been told by his exs that "im pretty sure you dont even know half the things hes done".. it kinda puts me off and leaves me blank and repent but them ive told her that it was the past but for some reason i just cant get it out of my mind :/ any advice? x