What does abuse mean to you?

I know there's physical, verbal, mental abuse and much more but something has been recently happening with my boyfriend and I and we've been arguing every single day for this last week to the point where I get so mad and upset and start crying. We've been together for 3 years now. Well I was watching my niece when we started arguing again and I tried walking by to check on my niece when I was just so angry he grabbed both of my arms and kind of held me still. I was so furious, I just had to fight my way out of it to just get out of his arms when he kept telling me to calm down. What should I do? I can't leave because every time we argue till we're down to rock bottom, he tries to "kill himself". He says there's no reason to be living and he has no place to go and since I don't want to be with him in a relationship like this, he says he wants to die and all he calls himself is a piece of shit. He has no job, he dropped out of high school, and does nothing but smoke weed everyday. What do I do? I'm getting fed up with this but I don't want to risk him taking his life because he has tried to show me he can take his life no problem. I'm sure my relationship is all kinds of abuse but he has never put his hands on my besides trying to hold me down. Any thoughts, ideas, or just help. I don't know what to do. There's a lot more behind the story so please don't take anything the wrong way. He's done so much for my family and I before. I'd like to say he was such a good guy but now i don't know what's up with him.