Autism

Possibly, that's still not ops responsibility. Let me give you a few reasons and deeper explanations as to why.

My ex kept me in a very violent and abusive relationship because he would say, I'll kill myself if you leave me. Should I have stayed to protect his mental health? It's a very common abusive tactic for abusive types to use extreme emotional manipulation and abuse, to entrap a partner.

It's not any one person's job to manage another person's mental health and if you aren't trained you could be meaning to help, but actually be doing extreme harm, or enabling, especially in situations like this.

Safety comes from feeling like you're in control of your emotions. If you ascribe your ability to be safe on another person too heavily, it can cause an infinite loop of fight or flight to an extreme degree.

If you aren't in control of your own emotions, your safety and ability to emotionally regulate is entirely based on another person, you have split your own "self", and them leaving would feel like a part of yourself is dying. It incredibly harmful to both people involved.

Your emotions are not someone else's responsibility, ever. Your emotions are a message from a part of your brain that isn't able to verbalise or understand language, for you.

They are most often a warning signal, that if you don't listen to, becomes distressed, which drives urgency in delivering the message, so those emotions become louder and louder until you can't ignore them. This is caused by any type of emotional unsafety. Your emotions were designed to warn you theres a leopard in the trees stalking you, if you don't listen to them, they get loud. They can't actually tell the difference between there being a leopard stalking you and any other form in nowadays and time, represents an unsafe situation.

Ops partner needs intense mental health assistance to help rewire an accidental loop of fight or flight they're stuck in. It is not helping them for op to stay.

I absolutely agree this person needs care, and would highly suggest finding a mental health professional and care giver, if able.

But you also have to consider that op doesn't owe someone else so much that they should live in a distressing state, themselves. If op wants to end the relationship, they very much should be able to leave at any time. Not being able to leave, or owing to someone else's mental health and safety before your own is not an ok burden to put on someone. Whilst I absolutely appreciate you have a kind heart in thinking about the partner, op deserves equal consideration, and it's very easy to accidental skip over someone's feelings and needs, because they seem quiet and less in distress. And you shouldn't have to go into distress to be deserving of your needs. I would suggest this relationship is incredibly mentally and emotionally damaging to op, and they deserve to be able to leave and heal.

Op, I am so sorry for all you are going through.