I'm so sorry you have to be here but also glad. It sounds to me like you have really taken the time to step back and take a good look and things and really think about it. My husband too seemed to go off the deep end starting right before we had our first child. I also understand the sneaking. Although I know it hurts, perhaps think of the time apart as almost a "favor". It's a selfish one, but it gives you time and you will have help.
"I am terrified of being a single mother, even though my husband says he wants to help out whenever he can."
It sounds like you are pretty much a single parent anyway. A two year old adjusts quickly, and he will love having his grandparents there. But it always drives me crazy when a father says he will "help". A father is not a sitter or a friend stepping in "to help".
Do you have a good relationship with the parents? I'm sorry if I missed it. Do they know what is really going on?