TL;DR at the bottom.

I've been with my boyfriend for a long time. Recently, we've hit a rough patch. We're trying to work it out, but I think/know that for us to make things work he really needs to understand mental illness. The hardest thing is that he thinks he does already, and he does know some things, but I feel like he really doesn't understand certain things at all.

I know it's partially my fault. When I was growing up I didn't have the best relationship with my stepmother, to put it lightly. When my dad would go into an episode, he'd say some pretty horrible things to her/about her. I'm ashamed to say, I never felt bad for her when he did this and I told my boyfriend that he meant everything he said. That it was what he really felt. I think I just wanted someone to see her how I saw her and not think she was this perfect saint for stepping up when my mom stepped out. "She didn't have to raise you, but she did." That's the response I got from multiple people anytime I brought up any issues I had with/about her. Fast forward to now....

I've had my own episode. I said some pretty horrible things. I don't even remember half the **** I said, but my boyfriend does, obviously. I try to tell him that I didn't mean it and explain that it wasn't really me. He can't get it out of his head that I'm lying because for years I said that my dad meant everything he said when he was manic. (I honestly believed he did. I wanted it to be true so badly.) I don't know how to reverse what I've done.

TL;DR
I guess what I'm asking is, how do you explain mental illness to someone who believes they already get it because they've had second and third hand experiences (through me) for years?