After all these years, I'm not happy anymore but I'm worried if I leave he literally won't survive. How do I do this?

You have to do what's best for you at the end of the day. If it's possible stay in his life as a friend so he still has you as a support system. If you can't you need to do what's best for you, it can be hard when you want to be there for someone but you can't stay only for his benefit. It's not selfish, you got this girl.

It might seem like it's completely up to you to help your partner, but it truly isn't. Each one of us is responsible for our own mental health. If you aren't happy in your relationship with this person anymore, you shouldn't feel pressured to stay to help him. It's never okay to sacrifice your own mental health for someone else's. What you can do is encourage him to get help and support him through tough times. Be patient, and help him understand where you're coming from. Let him know that even though you two aren't in a romantic relationship anymore, you are still there for him when he needs it.

Someone else's wellbeing and health is nobody's responsibility but their own. I know it may be frightening - but please do what is best for YOU and your mental health. The sad reality is - most people are too busy helping themselves to help you, so help yourself if you can. If he shows any signs that are concerning, or that he might harm himself or others, please don't hesitate to contact emergency services, and make sure you talk to someone close to him, such as his relatives or friends, to make sure someone is aware of his mental state. Best of luck