Preface - I'm no mental health expert. But this is what I would tell a friend if they were in your situation:

You can take the L today and this week. Things in life can feel like mind-numbing pointless exercise sometimes, that is normal. Forcing a smile isn't going to do anything about how you feel, and turning to vices (overeating, drugs, etc.) compulsively will only create more problems that are difficult to get out of.

Start by coming to terms with your feelings and that it's a bad day or week. Give yourself time, space and a degree of lenience. You can be your own harshest critic. Write things down, complain to the aether, cry, whatever. Then, let your emotions be what they are and start focusing on what you can control. Even if work isn't going well, you can prep yourself today or tomorrow's meal, clean your living space, pick out tomorrow's outfit, do shopping, check your mail or something else you might have put off. If you're too tired, then you can work on fixing your sleep schedule. Today there are no bars to clear, no expectations. Get nothing done, or get anything done.

At your next full day off, try seeking out a small change of scenery or pattern. Take a walk through a nearby park that you don't visit often. Buy a different kind of some grocery item than your usual. Take a bath. Give yourself an opportunity to reflect on your life, what you are thankful for and what you want from your future, however small or big, impossible or possible. Where on your priority list is the approval of your manager? Probably not even on there at all!

I know you will get through this downswing. If your emotion can't be helped today, then all you can do is prepare for it to be even a tiny bit better tomorrow. Go get it, champ!