How long have you been dating your boyfriend? It sounds like he is focusing a lot on your past, which is something that some guys do in the beginning of a relationship until trust is established and they feel comfortable in their roles. But if this is something that is continuing past 6 months or a year, it might be an indication of someone who is overly controlling and can be a sign of someone who can become emotionally or physically abusive over time. It is not fair for any partners to judge or shame each other based on what has happened prior to you meeting and being together. You are not accountable to each other for anything you have done, as long as you have been SDI-tested and are being safe sexually. You do not “owe” each other any details at all about past relationships or partners, and it is not normal or healthy to focus on this or to talk to your current partner about previous sexual exploits. Most couples talk about these things very minimally, because it’s likely irrelevant to your current relationship for the most part and can cause the issues you have mentioned.
It is not shameful or “wrong” that you have had previous sexual partners. As you stated, this is a perfectly normal thing and you should not be made to feel ashamed or “less than,” even for relationships you might regret now. “Hindsight is 20/20” as they say, because you know so much more now than you did then. Everyone has relationships they regret and decisions they wish they made differently, including your boyfriend. If he is continuing to pry or request more details about your past relationships or sex life, this also might be indicative that he is struggling with his own self-esteem issues or feels insecure about himself or your relationship. This is something he needs to work on himself, preferably through meeting with a therapist and processing some of his own internal beliefs and issues.
His focus on this issue has caused you to question yourself and to feel guilt and shame about something that is perfectly normal and reasonable. I’m glad you know yourself and your self-worth; hold on to that. If your boyfriend can’t recognize your worth as well, he might not be the husband YOU’D want. What if you had daughters? Would he judge and shame them for having relationships? What messages would he send them about women and sexuality? If you are thinking about marriage, I would highly recommend pre-marital counseling to see if you can get past this issue and put down healthy boundaries around this topic and work through some of his trust issues and insecurities. I’m hoping this is not the case, but here is a resource to screen for other controlling behaviors that you should be wary of if he exhibits them: https://pairedlife.com/problems/7-Warning-Signs-of-a-Controlling-Boyfriend-How-to-Deal-With-a-Controlling-Relationship
I know you probably wrote thinking that there was something you could change, but I don’t see that you have done anything that needs changing. It’s normal to withhold information or even tell white lies to someone who is hounding you for inappropriate information. This is a sign that you felt unsafe in those moments and off-guard, which are also not great things to experience in a relationship. If you are comfortable sharing this with him yourself, I would encourage that. Otherwise, you could consider going to a couples-based counselor to help with this and with teaching some more healthy ways of communicating with each other.
I’m going to include some resources that can hopefully be helpful for you and your boyfriend. I’m so glad that you reached out, and I hope you two can get past this!