How much can I talk about other relationship issues?

So I'm having pretty big problems with Partner A right now, to the point where I genuinely don't know if the relationship will work out or not. I told Partner B we've been having issues, mainly just to apologise ahead of time if I seem a little glum or checked out.

I understand it's wrong to complain about one partner to the other. But how much can I really talk about these issues? I think both Partner A and I are partially at fault. I know there have to be some boundaries here, but at the same time, I'd really value Partner B's perspective and emotional support.