When someone emotionally abuses and gaslights you, they make you doubt your own perception and feelings. It's very scary and and agonizing.
Looking for evidence is natural - not necessarily to use against them, but to convince yourself you aren't crazy. To convince yourself that whatever they are saying about you isn't true. To convince yourself that just because they bullied you into submission doesn't mean you are weak.
Maybe it would be good to write a letter, but then hang onto it for a few days to think if you want to send it. You don't need him to acknowledge that he was wrong. You don't need any further contact with him. Going "no contact" is taking your power back, too. Never speaking to him again is taking your power back.
You of course have the option of pursuing it legally but I'm not sure it's worth the mental health effects it may have on you.
Whatever happens, don't second guess yourself. Trust your judgment. You were abused and it wasn't your fault. Nobody deserves that kind of treatment.
And if he said "you abused me" then please remember that my abusive mother tried to convince me that I abused her when I was a child. Logic has no place in an abuser's mind, yet they can be masters at convincing you it was your fault.
A good life is the best revenge. Never seeing him again is the best revenge.