My boyfriend has depression.. how do I deal with it?

I want really good advice that can make me understand on ways to help him. He's been struggling with it for years since his parents got divorced at such a young age, but about two months ago he was properly diagnosed and under medication. This week we had such a good week.. I could see he was really happy until friday that all of a sudden he didn't call or anything. I kind of confronted him asking him why was he like that on Saturday and he just told me it was nothing to do with me, but of course I took it personally since I was oblivious of what he might've been going through. Later on that day, at night, we saw each other and he explained what was really going on. I feel so bad because I acted so selfish and uncaring with him before. What am I supposed to do?? How can I deal with him having sudden downs like that that completely shut him down? I really want to help him because I truly care for him.