Is it selfish to be tired of everyone and everything being depressed around me?

No.

Here’s what you do.

When you feel yourself getting overwhelmed, you state “I need some ‘me’ time.”

Then turn off your cell, log out of your computer, lock your door, whatever it takes, and let everyone know you won’t be available for X amount of time.

When you feel up to dealing with people, meet them on your terms (go out to a favorite gathering place, engage in a fun hobby, devote yourself to an energizing and renewing activity of your choosing).

This lets people know the conditions under which it’s okay to engage with you.

If you sense someone trying to draw you into a depressing discussion or using you as their sounding board, say something like “I’m unqualified to deal with that issue” or “I can’t help you with that.” You’re not someone’s unpaid therapist or default shoulder to cry on.

It doesn’t mean you can’t occasionally do that. It just means you won’t devote the bulk of your time and energy to maintaining someone else’s unhealthy status quo.

Repeat as often as needed until friends, colleagues, and strangers get used to the fact that your time is as valuable to you as theirs is to them. Spend it how you wish.