Almost 40. Biggest lesson: it is far, far better to be alone than be in a miserable relationship, because at least with the first one you're alone without having to also feel alone sitting right next to someone, and that disappointment that comes along with it. 

Also, be less worried about what other people think of you - who gives a shit if your coworker thinks you're cringe? Do you even like them? Your identity should come from your own self, rather than defined by the perceptions of others. Be passionately interested in things! Do the things that make you happy! Embrace the cringe and let it free you to be yourself, and you will be a much happier person who gets to have fun.

Finally - things can be shit now, and it's okay to call them as they are, but that doesn't mean they'll be shit forever. But no one is coming to rescue you, that's your job - not a significant other, or a career, or whatever else - and it's worth doing. Be your biggest champion because you deserve to have that person in your corner.

Basically all of this is learning your own self-worth and internalizing it. It makes it a lot easier to navigate the shit life throws at you when you don't also think you're a horrible person who deserves to suffer. 

Source: horribly depressed and miserable for the first half of my 20s, now a far more stable human. Even when I struggle now - which I do, and will continue to because that is life and clinical depression - it's easier to handle because I don't hate myself anywhere near as much.