That was my first thought as well - this might be more than just anxiety - but if it started after your son's birth and her father's death, maybe it isn't a personality disorder or anything of that sort. I'm really glad she's seeing a therapist right now, that's all I can say. Hopefully if there are other issues in there, they'll come to light. Otherwise, it may just be that the combination of motherhood and the loss of a parent sent her over the edge.
It's been said before, I know, but I have to say it again: You've done an AMAZING job plowing through all of this so far. I have to say, my ex finally became too much for me with all her crazy, bitchy ways and I just gave up. I might be biased (having eventually backed out of a similar situation), but my first inclination is to tell you to make plans for being on your own.
I will definitely echo previous advice about looking into custody rights. My biggest concern is your son in this situation. If it were just you and her, I'd say just be done with her insanity right now. Love and compassion are all well and good, but there comes a point when you have to admit that you aren't going to be able to save her and you realize it's time for a little self care. Since you two do have a child though, that complicates things... so just keep your options open and do a LOT of research.
In the mean time, stay strong and remember that it's not you. Maybe sometimes you are doing things that justify her being upset, but overall this is definitely her issues spilling over on to you. Don't let that ever influence the way you see yourself, and please be careful not to let her beat you down into your own dark place.