Lost my twenties to depression. How do you cope?

I've spent the last 4 years enclosed in my house. I don't really have any friends, last time I had sex was about 8 years ago with my only girlfriend. I despise myself, I want to get better but each time I think about how I've wasted my last ten years I feel like disappearing. I feel worthless, pointless.

So many things I wanted to do, experience, I wanted to be such a different man, I'm just a manchild right now. Maybe not even that. I can't be a parent (because of how I am). I'll probably never be in a relationship again because I don't want to waste anyone's time. People my age look at my perplexed: "What is wrong with this guy?".

I'm so so alone.

Anyone else is/was like this? How do/did you cope?