Hi and welcome!
Defending ourselves is a very normal response. It just doesn't work!
I don't know how to validate his feelings when I think they're so out of line. I can't say I agree or apologize for what I didn't do.
It is true that his feelings do not seem to fit the situation. But, if you can accept that he DOES feel what he currently feels (without judging whether he SHOULD feel that) then that's a start. Non-BPD people can "deal with" their emotions - we can put things in perspective and stop ourselves from overacting. People with BPD cannot. So:
Step 1) accept his emotion.
Step 2) Ignore what he is saying! "You did X to make me feel bad!". He feels bad. He doesn't know why - it's 99% because he's broken - but he isn't mature enough to accept that. (People with BPD live with great shame - and see things as all or nothing. Accepting their own flaws means seeing themselves as 0% worthy. They can't/won't do it). Ignore the "you did X" part and focus on the "I feel bad" part.
Step 3) Find some empathy. Find the "grain of truth" in what he says. If he says "I can't believe you want to go out with your friends and ditch me. You don't love me at all!" - try to understand what it must feel like to feel unloved. Don't judge whether he
should feel that or not, but accept he feels unloved.
So putting it together: "I am so sorry I have made you feel unloved. That must be painful to feel that your wife doesnt' love you.". Don't JADE! You may need to respond to a few of his statements in this way until he calms a little. Then after he calms - insert some truth. "Seeing my friends helps me relax." and try to help him solve his own problem "How can we make it less painful for you?". (Offer suggestions if needed).
It takes months/years to get the right words and tone.
Hope this helps!