Hi OP, I think what you are trying to ask is how much your upset may be about your BDD-related sensitivities, and how much of what has happened really reflects badly on your partner or gives you ‘real world’ reason to worry.
My instinct is it’s about 80-20 honestly.
in terms of your partner’s role, (the 20%) it is foolish of him to bring this up at all, considering what he knows about your sensitivities. And it sounds like he went into some detail (though I wonder if you pressed him to, and he essentially found himself digging himself deeper into a hole?). Now this 20% responsibility is pretty low, and I am basing that on the other info you have given about him- that he is caring that he has upset you and is trying to undo it, that he consistently gives you compliments and appreciates you, that there is no mention of him repeatedly upsetting you about other women….. and that your sensitivity/ BDD challenges have not grown or augmented generally by being with him… which suggest he is not the true source of the problem. It is always difficult to assess a one off event though. If contrary to what you have said, he routinely brings up other women, despite it repeatedly upsetting you, or he routinely puts you down (even subtly ie negging), and you find yourself more insecure with him than you felt before, then this would suggest he is the true source of the problem, is a bad partner for you, and your emotional health would be better without him. That becomes a LTB situation.
In terms of your role, you have already used the word ‘spiral’, and it does sound like your sensitivities/BDD are playing a big role here. It sounds like you are not able to hear/absorb all the good things he says, while extrapolating and being unable to let go of the concerning things. This can end up putting a partner in a situation that feels impossible…. I’ve experienced this a bit. It can feel like trying to constantly fill a bucket of the person’s self esteem…. Putting loads of good things in (caring treatment, attention, compliments), but it never gets full. There is a hole in the bottom of the bucket, and all the good stuff just pours through. Anything wrong/bad you say though, blows the hole bigger and the effects stay for ages…. Meaning you have to work ever harder at filling the bucket, and even the good stuff stops having any impact/being believed…. Do you think it’s possible this is the situation you are in now? What are the things you find work to help you take a breath, repair your own bucket, and start absorbing what is good?
if it is useful, I don’t see any objective reason for you to worry about this woman. There are plenty beautiful women in the world. You may be one of them and not realise. Or you may not be. But either way, your relationship goes far beyond that. It sounds (based on the information you have given), that it is loving and caring. I think the greater danger to the relationship is to get stuck in the leaking bucket situation.
I think focus on yourself right now with self care. He probably can’t help right now - the self regulation needs to come from you. What helps you when you start to spiral? What do you need to self regulate? Once you have refound your centre, you will be more empowered to see clearly what his role has been, decide what you want to do, and sort through it in terms of the relationship.
I hope this is useful.