Exercise, exercise, exercise. Load a free exercising program on your phone. Someone once said, "If your mind is troubled, focus on your body. If your body is trouble, focus on your mind."
Go outside. Every person should go outside at least once a day even for five minutes. Even the best day is ruined if you don't go out.
Do something horny. I'm serious. Go read a hentai book or something even if you're not horny. Sex sells. Mind likes to choose sexual pleasure over thinking of bad things. I swear if they replaced anti-science bullshit with free brothels, people would be much happier.
Don't think you're bad. Think you're good. You're smart. It's smart and good of you to feel bad. You feeling bad is your mind's way of healing itself from the shitty environment. If you're constantly sad, it means the environment is constantly shitty. It's the environment not living up to your standards. It means you have enough common sense and pride to not like every shitty place you're thrown into. It means you're a correctly functioning, real human being. Only brainless robots aren't sad and feeling bad sometimes. You deserve better!
Focus on your body. Are you feeling too hot, too cold? Clothes too tight, too loose? Light. Blast as much light as you can. Surround yourself with light. I just read today that light can heal something. I don't remember what. But I put on a flashlight and directed it at my eyes for a second. It helped.
Magnesium and salt. Always take magnesium and salt everyday unless you get it enough from food.
All the other vitamins and stuff too.
Wash your teeth. I'm often irritated for hours until I notice my mouth tastes bad.
Eat food that feels good in your mouth. Bad tasting food can make you feel worse.
Smell? Does the room smell good? Smells often make us scared, anxious, irritated without as ever realizing it.
Surround your field of view with objects that bring you joy.
Take a shower. I've also read that studies say that hot-cold-hot-cold shower helps people a lot.
Buy a VR. Go there.
Look at things that you fear. I've noticed that looking at tarantella pictures and videos helps me. Play a scary game. Play a scary VR game. Might not help in all situations, so try a little bit at first to see if it helps and stop if it makes you feel worse.
There was a study where a group of strangers went to a park to laugh. On their eigth meeting they immediately became happy and started laughing the second they stepped into the park. Conditioning works wonders.
Give yourself a hug, pat your head, caress your arm. Scratch yourself behind your ear. Humans function like animals. One day I thought that if ear scratching makes my dog happy, it might make me happy. I was right.