I understand you feeling vulnerable - it's frightening to show weakness when you have a history of abuse, because your experience will have been that abusers exploit any weakness and use it to escalate the abuse.
But please don't call yourself stupid. There is no stupidity about asking him to stop. Please don't second guess yourself with crap like "Shouldn't have started in the first place", "Should have known this would happen", "Should have said something sooner"...
Try reframing it to positive, because there are a lot here.
- You recognised that you weren't comfortable in the moment, rather than disassociating
- You used your words to ask for what you needed - this is a really big step if you're used to having your words ignored
- You were then able to articulate your feelings and explain why you didn't want to continue
All those are really positive steps that you took.
Plus more positives - your twat-dar has not malfunctioned to the point where you've repeated history with another abuser. Your BF stopped when asked to and validated your feelings. He reassured you that he won't hurt you. All of those are also positives.
Are you worried that he will see you as "a victim" because you showed your vulnerability? I have had huge problems with this myself. However I have learned to look at it like: if I was in a relationship (friendship, romantic, family - doesn't matter) with someone who showed me their "weakness", would I think less of them for it, or would I feel happy that they trusted me enough to show me that part of themselves?
Of course, I'd be happy that I now understood them a little better. I wouldn't be sitting there thinking "HAHA, LOSER!" and I'm willing to bet my last Pot Noodle that you wouldn't either :)
You have done a GOOD thing OP. Please see this as a step on the road to building your inner strength, because it is. 