Depression Support Group
Depression is a real and debilitating condition that is often misunderstood by family and friends. Its meaning can range from a prolonged period of sadness to an actual mental illness with specific symptoms. Find and share experiences with others who are going through the same struggles.

My guess is that once you and your friend realized his limits (about discussing suicide) the conversations had zoomed way past his limits. Now he is dealing with that overload.
By all means stop apologizing, well perhaps one more where you announce calmly that you are sorry the conversations bothered him, but that it is over and yo will not apologize for it anymore.
You've broken up. He doesn't want to come to you with anything. He's wanting you to go away.
It hurts to learn that you have actually ruined something and can't get it back. But you have. Never date anyone till you fix the depression. Anyone who likes you in that state is a bad guy. And he doesn't like you in that state.
Ask yourself why you are so invested in turning him around and having another chance with him. I think you'll find it's to do with you and your self esteem.
Being given the kick is a terrible blow and it can take a while to realise it's happened.