I think Justin was lucky Brian took him back... but not because Justin was in the wrong and Brian was a Saint. They both fucked up and I agree with you that in their own way they both acknowledged that. We KNOW that Brian was desperate to have Justin back the whole time he was gone. I think Justin was lucky that Brian let himself take the risk of opening up to something that scares the shit out of him again. The first time he let Justin become part of his life it was sort of an accident that happened over a period of time, but in 308 Brian knows what he is getting into.
But no, I don't think Justin would say that and Brian would think it was complete bullshit if someone said it to him.
That said, it has been so long since I watched the show that it almost doesn't feel right to even answer this question!