I wanted to add that I am no expert...this is very new to me too.
That said, the fact that our primary relationship survived one of his girlfriends also gives me enpowerment towards security and trust, that nothing can destroy us, but us.
I saw that he was able to love me, please me, be my lover/partner with no problems even when he was with her...is this the case for you? Do you feel he is not fully what you need because of the other boyfriend?
What are your fears? Why are they your fears? Answering these questions may help too