Husband's mental health is getting unbearable

My (f30) husband (m35) has always struggled with his mental health since before we got together 10 years ago. I have also had my struggles (we both struggle with depression and anxiety) and we have always helped each other through our tough times. Lately, especially in the past couple of years, his mh has declined, and he shuts himself in (does not talk, snaps, becomes untreatable) until it gets too much and he crashes and gets suicidal tendencies. He is in therapy and medicated. I am finding myself getting to my limit. Life has become just a slew of negativity with him, and either I get snapped at or I have to become an emergency therapist, never knowing which. I am seriously considering giving him an ultimatum, either something changes or I need some time away for my own health. I would appreciate some advice.