Having gone through a few of this and anticipate going through more in the future...
1. Find something to look forward to. Wether it be that new video game coming out or there's a movie coming out you want to see. Heck, even if it's just because Christmas is coming up and you love seeing the twinkling lights. Or getting another paycheck at work.
2. Wear your favorite item of clothing. Even if it's the socks with the frogs on them.
3. Remember to eat and stay hydrated. Trust me, dehydration sucks. You do not want to go through that.
4. Keep on top of your hygiene. Yeah. I know it may seem hard to get in the shower but even something as simple as brushing your teeth or washing your face can make you feel better. Even if that's the most you do in a day. You can feel like you accomplished something by washing your hair.
4a. Don't feel like taking a shower? Soak in a warm bath with bubbles. Use that fancy bath bomb you've been saving. Or use some dry shampoo. Get some nice smelling lotion. Winter's coming up guys. Dry skin galore.
5. Treat yourself. Have a bowl of ice cream or buy a new shirt. Make your favorite meal ( or ask your cooking parent to do so for you. )
6. Watch videos of baby animals.
6a. Listen to nature sounds. Bird song, whale song, rain. Around this time of year when it's cool enough, I like to sit with the door open and listen to the leaves rattling down the road.
7. Listen to uplifting music. You're going to be tempted to listen to depressing music because that's the mood you're in but it's not going to help. I personally listen to '80s music because a lot of it tends to be upbeat and peppy. Dance music also helps.
8. If you can, sit outside for a bit and enjoy nature. Walk around your block or your college campus. You get fresh air, sunshine, and some exercise. Winner winner chicken dinner.
9. Find a Wiki and keep clicking ' random page '.
10. Can't play a videogame for whatever reason? Watch a Let's Play of it. Sometimes you get the added benefit of quality commentary. I'm watching this LP of Minecraft right now and it's one of the best things ever.
11. Find a communication outlet. Friends, family, your favorite teacher at school, a co-worker, someone you can talk to. ( Because let me tell you, isolation is one of the worst things you can go through. )
12. Keep up your sleep. You need sleep to function. You need sleep to heal. Just like from a physical injury or an illness. This is no different.
13. It may seem a bit hollow sounding, but things will get better.
14. Clean. Even if it's just one small portion of your bedroom or you clean out your purse/backpack. It's best to do this on a day where you can open the windows. Again fresh air.
15. The best thing is when the gloomy thoughts start creeping in is to occupy your mind elsewhere. Try doing a crossword puzzle or a word search. Play a board game with a friend or relative.