Yeah, I experienced a similar incident. It sounds to me like something had happened to him while you two were dating that was very triggering to him, and started the downward spiral. At least that's what I got from what you said.

He probably feels like he's damaged beyond repair and that you deserve someone who isn't "damaged". That you deserve better. Also, remember that you can offer all the support to him in the world, but only he can decide to get help and try to get better.

Take care remember to not be so hard on him or yourself. Depression is a nasty thing to deal with.