I'm so sorry for what you're going through right now, and have gone through in the past. I can understand why you would put yourself down or wish you had never even given birth to your son; however, as various mental health professionals have already told you, dwelling on the past won't change the relationship you have with your son now.
Please try to be kinder to yourself. Yes, some things may have been in your control; however, when faced with an abusive relationship, especially with one where the abusive partner is capable of murdering you (as he did his third wife), it's important to proceed with caution. There's a reason why so many abused partners make several attempts before they're successfully able to leave the abusive partner: there are real dangers to leaving, if you don't have all the safety measures in place! It's easy for people to say, "You could have done better," when they haven't been in your position... that includes your son.
Speaking from the perspective of an adult child, I want to note that what your son is doing is not entirely because of what you did (or didn't) do. Many people witness domestic violence while growing up, but not all of them shun their parents for failing to change the situation, and not all of them have "messed up" lives. Some of this comes down to decisions your son made. Instead of choosing to reconcile with you, he chose to push you away. Instead of choosing to include you within his network of support, he chose to exclude you. I'm not trying to point fingers at anyone, because I can understand why he would blame you for some of the pain he experienced; however, YOU didn't cause the pain. Your ex-husband/your son's father did, and just as you and your son had choices, that man also had choices, and he made bad decisions when it came to caring for his family members.
I know this one post can erase all the pain or ease all the feelings of guilt/regret/shame/etc... but I hope you'll believe me when I say that you are a person of value, and you deserve forgiveness - if not from your son, then from yourself. Being able to forgive oneself is a very difficult task, perhaps even more difficult than being able to forgive others. I hope you'll continue to strive for that self-forgiveness and move forward knowing that, while you can't change the past, you can work toward creating a better future for yourself.