You know, it fascinates me how men can make women feel like it is their fault that the man did something wrong.
One woman I recently talked with was convinced that it was her fault that her husband hit her.
You boyfriend wants to convince you that it is your fault that he slept with his ex girlfriend. He may even believe what he is saying. . .doesn't make it true.
Communication is obviously lacking with the two of you. So perhaps that is a good place to start.
It may seem corny but I think it could be wise to consider outside help, like the book 'relationship rescue' by Doctor Phil McGraw. It is basic simple information that helps couples to communicate better and to figure out who they are if they are compatible and going in the right direction.
I mean, good grief, we get training to drive a car, but not to be married and have kids together??? Don't just try to figure this out with your limited experience and his. Find some safe places to learn from others - such as Dr. McGraws books.
Someone once said, a smart person learns from their mistakes, but a wise person learns from others mistakes (and a fool never learns).
Also, this may not seem real to you, but praying for God to lead you into the truth in this is a really smart thing to do. He knows this mans heart inside and out and knows if he is honest or is someone who is going to eat up alot of years of your life. There is an old song about it being "hard to belong to the wrong one, when the right one comes along". I'm 55 years old and I have seen alot of women with the wrong man when the right man came along. Believe me it is horrific. It pays to pray and say a simple thing like, "If there is anything hidden here that needs to be revealed, please reveal it." And "If this is not the right person for me then please make it obvious."
Jesus said if we opened our hearts to Him and received Him that He would guide us into all truth. Just something to consider. He knows your heart and He knows who's heart best matches yours.