You sure have been through a rough ride in life, but I'm confident you'll work things out.
Now first, you're absolutely not responsible for your ex-hubby cheating on you, that has been his choice not yours, and he harmed you with it and believe it or not even more so he harmed himself and so did his ex.
But you did choose to stay together with him when you found out that he cheated on you while you're 2mths pregnant. You could have called it quits then and even theoretically got an abortion. Then you found out he cheated on you the night before birth. Twice the universe told you that your hubby didn't really love you the way you wished for, and twice you chose to stay nevertheless.
You do need to take responsibility for those choices and where they have led you to suffer more than you might have otherwise. You rage against him and life, and that's okay cause the wounds inflicted are still fresh, and anyway it's better than being depressed, but don't stay stuck there. In a way by blaming him, his girl friend and the universe, you still feel crappy cause you still don't take full responsibility for your life.
Sometimes we know things deep down, and still we try to pretend to ourselves about them for various reasons, but in the end truth prevails. When you did leave you felt fantastic cause you finally acted on the truth you've felt for so long, you were true to yourself and took off the self-imposed blinds. In a way you betrayed yourself all that time you stayed with him when you felt deep down that he didn't really love you the way you wished for.
I don't believe your ex ruined your life or the life of your son. I believe it would have been ruined had all gone according to plan and he'd stayed with you for the sake of keeping the family together when his heart was elsewhere. No true happiness can grow out of a dry and bitter sense of doing one's duty, only a sort of grim satisfaction comes with it. Would you have truly wanted to live that sort of life?
Now both of you are free to find the fulfillment you're really looking for. And even if it may not look like this right now, I believe you have a head start in this, cause you didn't do things half-heartedly, you didn't cheat, you made choices and took action. You're definitely not a failure, that's just the same voice talking to you that told you to stay on when your gut told you to get out all along.
You always have choices, and it helps to remind yourself of alternatives at times to validate the choices you made. When you say you're a single parent, theoretically you could have left your son with your hubby. Would you truly have prefered that to the status quo? And you do face challenges right now in trying to look out after your life on your own once more. There are support groups out there for single parents who can help you not only with the anger and pain, which are feelings that naturally follow in the wake of being wounded, but also can help you with the practical concerns of everday life to take some off the stress out of it.
Finally you say you're depressed when you think about it all a certain way. Well, I'd say this points out to you that those kind of thoughts on the issue take you further away from who you really are. Consciously try to think differently about it and pick the thoughts that make you feel better. Whatever thought makes you feel stronger and better about yourself is a good thought and in tune with who you truly are. I think there's a good book by Esther Hicks on the subject of emotions, and I believe it's on Steve's reading list.
You've shown all the qualities it takes to make it in life, so I'll say it once more, I'm confident you'll work things out.