Angel,
I can definitely sympathize with your situation. My husband has suffered from depression for most of his life, although he wasn't diagnosed for many years. He tried several different meds, but a combination of Wellbutrin and therapy helped him. He has been off his medication for approximately 3 years and is no longer seeing a therapist. He has had experiences similar to your husband in that he has had small bouts from time to time, but is now experiencing a backslide.
I can share in your frustration and anger. We now have a four month old and more than ever I feel that I am less helpful and sympathetic to his condition. I have to keep reminding myself that it is an illness, just like diabetes or heart disease, etc. Unfortunately, as you and I know, the illness itself sometimes makes it hard for someone to seek help. I know my husband has a horrible time asking for help and would rather do everything himself. He feels that going back on meds is a sign of weakness and just makes him feel worse about himself.
As for whether or not leaving would help, it may or may not. It could have the opposite reaction and just solidify his feelings of poor self-worth, which may not help the relationship recover. However, my husband and I had a counsellor (who was horrible btw) who suggested I should leave him so he "could learn to stand on his own two feet." This fear did seem to motivate him to continue seeking help and he eventually found a solution that worked for him. Ultimately though, I think you need to do what is best for you too. If he isn't willing to get some kind of help (meds, therapy, etc.) maybe you need to consider leaving.
Sorry this post was so long...I need to learn to be more concise. Hope it helps to know someone else shares your frustration though.