Recently I had a bit of a family crisis involving my nephew. He hasn't had a terribly good life. The situation he was in was remarkably like a situation I was in at just about his age. I was trying to help him, but had a horrible time divorcing my own memories and feelings from his current problems. His problems were not my problems, no matter how similar. What he felt was not what I felt.
It sounds like you may be projecting your own inner (and unfulfilled) child onto your own child - thus the envy. The "why couldn't I have"s.
I envy dogs. Dogs live in the right here, right now. Often, when I find myself slipping into old hurts, I look at my dogs and remind myself of why they're so happy all the time - they don't harbor grudges, they don't worry about yesterday or tomorrow. I try to focus myself on the right here, right now. I try to let go of what was in the past for a moment, and concentrate on what is in the present.
You can not change your past. You couldn't change it when it was the present, and you definitely can't change it now. Like you, birthdays are difficult for me. I changed them. I found a way to make my birthday something special for me that didn't involve any old triggers. Perhaps you could do the same with your son.
Another idea would be to have a (secret) double-birthday party. Throw him the birthday party you wished you'd had, then partake in that party like you were one of the guests. Pin the tail on the donkey, eat too much cake, play video games even if you're bad at them (especially if you're bad at them, nothing boosts a child's self esteem like beating their parents at something!). Nothing says that just because you're the adult, you can't play also. Remind yourself that your parents couldn't (seriously, in their cognitive functioning state, they really couldn't) give you what you wanted, so you're giving it to yourself - a little late, but better than never.
Try to remind yourself that how you got to where you are now is not nearly as important as what you're going to do from here. We can't change the past, we can change the future. It is no longer your parents who have control over what you do or how you live in this world. It is now entirely up to you.
As for recovery, I don't think recovery is a destination, I think it's a journey. I don't think anyone is ever "recovered" - we're always working on it.
Wishing you the best for your son's birthday, and I'm very proud of you that you are working so hard at breaking the cycle. It takes much more effort to swim against the current than with it.