Oh God, girl I feel for you!
I was in a very similar situation myself many years ago.
A narcissistic ex, whom I had found a way to keep in control. And I was in a relationship with a Really Good Guy, all heart and helpfulness. All was well until the ex kicked off.
I handled it my way, but the narcissistic rage was strong and we were getting to the point of legal action (I had said 'no' to ex over a holiday arrangement that he left until the very last minute, and I genuinely had no flexibility to say 'yes; and he spitefully started threatening, through legal letters, to have the child in question taken into care because I had been abusing her).
Really Good Guy let me handle it my way for a time, but was all 'why can't you just be reasonable? talk to him reasonably?' like it was at least partially my fault.
I tried to explain - narcissist rage, punishing me, no room for reasonable - but Really Good Guy didn't see it, and thought I had just taken an entrenched position.
Really Good Guy took it on himself to get in touch with the ex and arrange to man-to-man chat to sort it all out. Told me he intended to do this. I asked him not to, explained all the reasons why this would backfire - I'm not going to say I 'begged' him not to, but I made very strong representation, asked him in the strongest terms not to.
He went anyway.
As far as I was concerned, that was the end of my relationship with him. He came back, crowing about how well it went, how reasonable ex had been, how all he needed was someone to talk to him equally reasonably....... I said nothing.
Half an hour later, we got a fax (ex's preferred medium of abuse) extending his allegations of abuse to include Really Good Guy kicking her.
Really Good Guy finally got it. Profuse apologies, and he has had my back with ex ever since (married 25 years now).
So I understand where you are coming from, what you are going through with a narcissistic ex and a Really Good Guy who doesn't understand, even when you explain. If my ex hadn't have sent that fax, I would have split with Really Good Guy that night.