Hello both of you, and welcome
I am in the same boat as both of you. When I first came to PC, I was desperate and in retrospect, depressed myself due to the stress and anxiety of living with someone with severe depression. Thespouse... I have written almost identical posts as yours, many many times. LovesEcho asks some good questions... but I also understand your concern about leaving and worrying if he was going to get worse as a result.
My husband's depression is managed and under control right now, and I am eternally grateful for the support I have received, as well as for him because he eventually agreed to seek treatment. It wasn't easy - I feel that I lost years of my life while he was severely depressed - but I do have to say that I'm glad that I stuck with it because when he's feeling well, he is a terrific partner.
Please don't feel guilty about anything you said. Spouses/SO's of depressed people are an overlooked population and we have support needs that many cannot possibly understand, especially those who are depressed.
I am here for you if either of you ever want to talk.