Hi there,
there's no easy answer. We would need one for every ocassion.
Reasoning and even presenting evidence doesn't work.
False accusations hurt a lot, and the rejection that follows, even more.
I think one short answer is needed. "I'm not cheating, and I have no intention of doing it". And then walk away or wait for it to pass.
It will surely come back, but arguing just makes it worse. You can have false accusations + fight. Or only false acussations.
Those accusations are offensive. If not abusive. And your goal is to not be exposed to that.
Let's say your SO has a nervous tic, he suddently lifts his arm. If you are around, he would slap you, accidentaly. He can't help it, but you can give him a step when the tic manifests, and not be slapped. If he did, we wouldn't be offended, but still hurts.
We want to get to a point when we know its not personal, we did nothing to provoke the accusation. So, as it's said often, it would be the same as if they told us "I think you are a pink elefant". And we have to take it like that. We know it is false and absurd, we don't have to argue that we are not pink elefants. I manage to do that one of every 100 times, and I hope to improve those numbers.

There's gonna be time outs, times when he puts some distance, and we have to learn to live with that. As we need time to focus on ourselves, that's the perfect time to practice that.
Here's a lesson on that. This is a very common trait for most people here:
https://bpdfamily.com/message_board/index.php?topic=78324.0Hope you learn to cope with that, normalize it and not let it affect you. With time it doesn't hurt me that much.
Brothers and sisters, don't be shy! we all deal with this, chip in and give us your two cents!
