Dating someone who’s mentally ill - where do you draw the line?

My (29F) partner (31M) is a wonderful, perfect partner 29 days of the month. The other 1 or 2 days, he has a mental health crisis, and it’s awful.

We’ve been together for three years. The first two years were fantastic - he’s kind, thoughtful, considerate, and I had never felt so confident about a person. We have the same values, want the same things, and I was sure we would get married. We also live together.

About 9 months ago, he started having spirals. Usually triggered by intense work stress, he’ll become super emotionally disregulated and sort of lose his sense with reality, to varying extents. He’ll say things that are very dramatic that he usually renegs on later, cry, become fixated on EVERYTHING being bad (our relationship, apartment, money, work, etc.). Typically though, it’s all very self-depricating. He doesn’t focus it too often on me (though it’s happened sometimes) - it’s more him hating himself, wishing he wasn’t mentally ill, etc.

The spirals have been getting more severe. A month ago, he had one where he was hitting himself in the face repeatedly and gave himself a bloody nose. It was really scary. I had to leave for work (I told him I’d stay, but he said me staying would make it worse and insisted I go), and I was honestly terrified the whole time that I’d come home and find him dead.

He then proceeded to be absolutely wonderful for a month, and then the same thing happened last night. Intense work stress, declaring he’s going to quit, deciding maybe he shouldn’t quit, then hitting himself in the face, nosebleed, falling on the floor, suicidal ideation, etc. He was back to normal two hours later. We kept our plans to get dinner with friends, watched sitcoms, and went to bed like nothing happened.

I feel safe, and I’m confident he would never hurt me (also tbh doesn’t have the physical ability to), but it’s all incredibly emotionally harrowing. I’ve lost so much confidence in our relationship, and though I still really, really want the future we’ve planned, I don’t know how he’s going to be able to handle having kids if he’s reacting this poorly to work stress. 

He recognizes he has a problem and is in therapy, but… I don’t know. It’s not enough. He’s been in therapy for around 5 months, and I don’t realistically think talk therapy is going to fix what’s going on in his head. I’d like him to see a psychiatrist, but he had a traumatic experience with one when he was younger and desperately doesn’t want to go.

I love him so much, but so much confidence has been destroyed. I’m constantly overthinking things, and I’ve become so calm all the time because his big emotions always take priority. I want the version of him from a year ago, and I don’t know if I realistically can get that person back. 

I’m also a little worried I’m becoming desensitized to the meltdowns. Last night, I didn’t cry, I didn’t get worked up, I just calmly helped him through it as best I could. So my question is, when you’re faced with a terrible situation like this and have been through it multiple times, how do you decide when a final line has been crossed? If you’re still staying (for now), how can you set aside all the constant doubt and work on gaining confidence back in the relationship?

tl;dr - Partner is great, but mental health has been declining for 9 months. How do you decide how much is too much? Until I reach that point, how do I balance supporting him with coping myself?