Well, for me I don't know if my parents did or didn't do their best, I don't think anyone can speak about someone's potential (it reminds me of staying with A bacause we know what a great person he/she can be), I mean we can speak about it but what does it really mean?
But I know that for some reason or the other, because of their own circumstances, bacause their own caracter flaws, their own deficinences my parents couldn't do better.
I think there is a big difference between doing (or not doing) your best (as that kind of implies choice) and not being able to do better.
The second doesn't excuse the behaviour either, but it is much easier to forgive that way. And that forgivness is about helping me to cross that bridge to healthy and happy life.
Well, that's how I see it.