This is just a possible explanation.
You may just have had an "amygdala hijack."
"The amygdala is the “fight or flight” and emotional memory part of the brain. Its job is to protect by comparing incoming data with emotional memories. An amygdala hijack occurs when we respond out of measure with the actual threat because it has triggered a much more significant emotional threat. For instance, the amygdala will react similarly to the threat of being eaten by a tiger (physical threat) and the threat of an ego attack (emotional threat) by bringing on the fight or flight reaction.
When one experiences an amygdala hijack, the amygdala overtakes the neocortex (the thinking part of the brain) and there’s little or no ability to rely on intelligence or reasoning. The effect is that energy is drawn exclusively into the hijack. The immediate result of a hijack is a decrease in working memory. Adrenaline is released and will be present and effective for 18 minutes, and other hormones are released into the bloodstream that will take 3-4 hours to clear."
So your boyfriend may say "What did you do today?" (or something innocuous) and your emotional memory pattern is triggered based on something you make up or that happened to you years ago and you get hijacked. You react to a question as if there is a literal threat of violence. Heart rate goes up, muscles clench, hands sweat, breathing shortens... and it was just a question.
That might be part of it.
The other part of it, is that you simply don't have an emotional memory pattern that is consistent with being in a loving, vibrant, creative, supportive relationship. So when that is happening, your brain wants to normalize the situation and to get you back into a situation where it "knows" what to expect... i.e. drama, being alone, being miserable, etc. Your brain knows what all of that is like and "it" (not you) would choose the safety of the "known" versus the risk of the unkown.
All there is to do is notice you're getting hijacked, acknowledge it, give it up and create something. "Wow, he said "how was your day?" and I was immediately upset, like he was gonna get me or something. I just got hijacked. Well, that's not what I'm committed to. I'm going to create being courageous/creative/sensuous/etc. instead."
Now, you are at choice in your life... instead of being driven by your lizard brain (the amygdala.)