Before trying to forget the past of your boy there is something that you must have very clear and it is that past you have remained in the past, that does not affect your present.
If on your part the doors of the past are closed, you have not right to open them. So, what to do?
1 Do not try to forget it because it will become a recurrent and obsessive thought that will not let you live.
2 Accept it. Accept your past and your mistakes but do not carry them. He is in another stage of his life and different (we assume) does things differently.
3 Talk to your boyfriend until everything is clear. Do not avoid the subject to not make him feel uncomfortable. If you need it, a good communication will make you have all the information you need to feel calm.
4 Watch your boyfriend. Does your boy look like that person who is the protagonist of the dark stories of his past? Does he have the same attitude or do you not recognize him in his account of his past?
5 We all change or evolve in life. No, nobody can make change and you will not be able to change that man. But that man has been able to change by himself, maturing or learning from mistakes.
6 I’ve got another chance, like everyone else. But you are the one who has decided whether to give it to him or not. Do not go on with a story when you have not really forgiven your past.
7 And think: are you happy in your relationship? Because if you are happy, if you love him, if you see the future of your relationship, why are you going to destroy the present moment by matters of the past?