My bf doesn’t trust me bc of my ex

So my current bf and I have been together for 2 years. And my ex is an absolute thing of the past. Well 2 nights ago while my bf was laying next to me I get a notification that my ex had added me on Snapchat. My first instinct was to panic because I am human and while I wasn’t going to, every part of me wanted to bitch my ex out because he is 26 years old and cheated on me with a 17 year old fresh high school graduate. I wasn’t going to do it, but when I saw that he added me I just simply exited out of it to process. Well my boyfriend saw this and got mad that I did not immediately block him and is questioning why he wasn’t blocked in the first place. My reason, was that I’m sorry and I know how it looks but I panicked. That’s the truth. And to why he wasn’t blocked in the first place, he unadded me when we broke up before I could unadd him, so I could not block him, I didn’t have his username memorized to look him up and block him. He’s blocked on everything else. Now my boyfriend doesn’t trust me. Thinking that my inaction was action. And how was he supposed to know if I was saving it so I could add him later and message him.

I get his concerns, and I have acknowledged them and apologized. But I hate apologizing for this because in my mind it feels like I’m validating and apologizing for adding my ex and not for immediately blocking him. I am sorry that I didn’t block him in the first 5 seconds of seeing it. And I’m sorry that he’s having problems trusting me. But I’m not sorry for the narrative that I was going to cheat on him because I wasn’t.

I need advice on how to apologize and slowly regain his trust without increasing his opinions that I was going to message my ex.

I feel like I’m being punished for needing a minute to think and that doesn’t make me a cheater that makes me human. I never want him to doubt us, but idk how to apologize for a potential scenario that didn’t happen.

Right now I just apologize when he brings it up and validate that I should have blocked him in those first few seconds.