I have been in almost an identical position. When I got together with my partner 8 years ago his mental health was bad , he never acknowledged it and said it was just him. I made him go to the doctor and went with him . He was diagnosed with depression and put on medication . Over the last 8 years there has been times where he has forgotten his medication and been abusive , blames me for triggering him etc. I have always felt like I was the one causing his bad times . I asked him to get help but he always put his depression down to the fact he had a chemical imbalancement. Fast forward five years, we had a massive fight he was the worst I had seen him and I was tired and exhausted . I gave him an ultimatum, like you did . He got counselling and has since become better at understanding his depression.

It won't be easy but you need him to get help and you need to know that as a partner of someone who suffers from depression you need support yourself . It has taken me six years of just dealing with it and pushing it away to finally realise it has effected me greatly also.

What I have realised is people with depression go through a phase of denial and responsibility .regardless of where he sits it's unfair on you and you can only ask he gets help .