D.M.
To break the cycle of the convesations you could be direct and say something like - you're right, you weren't always the best mom (basically agree with her). Then go on to say that you know she did the best she could, and love her and wouldn't wish for another Mom now. We all learn as we go and do the best we can. Now it's time to move forward. Don't harp on it or feed her guilt. Just acknowledge the truth and forgive her and move on. If she wants to talk it out you can, but I wouldn't get into excuse making, just process, forgive and let go... move forward with now.