If he pushes you away when he's depressed there is not a lot you can do aside from talking to him when he's not depressed about how it impacts you. Maybe you both can come up with a game plan for when that happens. I admire your tenacity in not letting go. Can you give him the space he needs when he is depressed? I'm sure your affirmations and reassurances are important during that time but he may simply need space.