"I feel like I have seen and known too much already to ever become mentally healthy"

You are a realist. Not a pessimist or an optimist, but you look at things and see them as they are. It's a proven fact that people who are depressed see things more realistically. Who wouldn't be depressed in this disgusting world? This world is one step above hell, if you ask me. It's normal to see this ugly reality and be depressed. But you must realize that this world isn't all there is. Focus on working on yourself and helping others. Focus on loving and follow God. It's the only reason to live.