Sorry for being abstract and dramatic.
Suppose you're 31. Suppose you have wasted 5 or so years, completely, to the point where you no longer recognize yourself as a person. You don't know your place in the world anymore. You have no value whatsoever.
You don't know how to start life anew because your life is in the past. Your friends, your colleagues, your parents, your sense of unity with the world – it's all in the past. You can't reconnect with it, because it doesn't exist in this moment or in the future – only in the past. And as it turns out, you simply can't exist without this sense of connectedness.
You can't really imagine a future where you are content, because there is no "you" anymore. This future is reserved for the version of "you" who lived never being in this depressed slump, who managed to keep his mind clear. Who never lost connection with the world, because once you lose it, it's impossible to come back.
You can't read anymore. Characters are fulfilled human beings, and you are not. They participate in life, they make decisions and have their own agenda. You can't relate and it's very scary.
You are 31. But you feel 25, not because you feel youthful, but because time stopped for you when you were 25, and the world has moved on. You can join the world again – but it's not your world anymore.
You've been thinking, thinking, thinking countless hours, but you can't fight your instinctic feelings. It's really over. Your life is over. There is no new one, everything about it is fake. Even if you somehow get better, you know now what kind of human being you are deep down. You feel disgust and shame at whatever you do, even walking or eating. Because you are not human anymore.
What do you do then?