I'm actually going to go out on a limb here and say something almost completely opposite: You can't fix someone with depression. You can't fix anyone with mental illness, period. In a relationship where one person is struggling with mental illness, the partner is the one that has to decide whether the relationship is worth it and whether they will actively fight for it because there will be very difficult times ahead, that will try your patience, hope and faith.

My advice to you would be to sit down and just read a few articles on depression. This isn't much, but something you can do to release some tension and anxiety on your end. I can guarantee you, you'll find about 85% of what you've written as directly related in one way or another. So the good news is: this is honestly nothing new. Some people don't want to communicate when they're this way. No matter how much he might love you, he'd rather deal with this by himself. Or he can't deal with it - simple - so he's coping instead. Bottomline, read up on depression and then make the decision whether you're ready to be in such a relationship (like with any relationship), because this is just the tip of the iceburg.

What is cause for worry is his complete disregard in communication about what he's doing about it. Because re everything above, it only works if the person is actively trying to improve and deal with the situation. Otherwise, it does become unfair to you. You do have a right to tell him how you feel, and throw down an ultimatum should he not do anything to actively change the situation. And don't be afraid to do this. This is a key point in making it work. He can deal with it however he wants to (and yes, it may possibly be without your input) but he has to be going to a therapist, he has to be taking medication, he has to be trying, otherwise you're the only one who is. Note: Personally, I would not be okay with the answer that he's "dealing with it himself". He needs professional help and you need to demand that of him.

Do not his take depression personally. Do not. It will wreck havoc on your own mental health and you will sit in a puddle of these kinds of questions for eternity.

You've known this man and loved him for 4 years, though. You know the type of person he is: if he was treating you sub par the entire time and only now it's this horrible, then why break your heart. But if you see good in him, and you know he's an amazing person, then you have to make a difficult decision, because depression is...hard. On both people.

I hope this helped, and I'm sorry you're going through this in such a way.