You have a right to your privacy. It is normal and appropriate for teens to explore a number of relationships before settling into a lifelong commitment. It is how people discover who they are in a relationship and what kind of person they want to be with.
Your boyfriend’s behavior most likely is the result of some insecurities on his part. Perhaps he worries that he doesn’t measure up to another guy you were with. Perhaps he is anxious that you might leave him. Or maybe he is worried that you have had more experience than he had. That shouldn’t matter, but it may matter to him.
You two have a young child together. It’s very important that you get through this for her sake as well as for your own. You can’t go through life essentially locked up. He can’t go through life feeling so insecure. I strongly urge you to get some couple therapy to help the two of you resolve this issue and to explore why it is that you haven’t married in spite of being together for years and having a child together.
I wish you well.