Mutatio, How can you feel better? How can you, as a man, not as a beaten down husband who screwed up in the past (and the distant past at that, it is ancient history), but as a man with a future, do better for yourself? I know you are filled with sorrow, so am I, more than I'd like to admit. I don't know how to make that go away except with time and moving forward.
I know you blame yourself, so did I for about 8 months and it was torture. Yes, you had a role in the demise of the marriage. Yes, maybe a big one. But that is over. You have accepted responsibility, changed yourself, you are NOT the old drinking, bullying, Mutatio. Forgive yourself. Let go of the guilt, it served you well as a catalyst for change, now it is keeping you stuck.
Your son needs you just as much as he needs his mother, but he also needs you in a different way, at this point in his life he might be closer to his mother but your influence might actually be greater. You are his role model for what a man should be. You are teaching him how to relate to the world and to himself. You are teaching him love and commitment and patience. But you are also teaching him by example to be hard on himself, to hold onto blame, to feel guilt. So if you are staying for your son, I think the best way to really do that is to let go of your guilt and find a way to be the man that you have worked so hard to become. Find a way to be content, find a way to forgive yourself, and to be confident that YOU are a strong and loving and amazing man who has a lot to offer to the world.