The best advice that I can give you:
Exercise. When I feel down, it helps a lot to do short runs. The positve effect of sports is well established and can be felt almost instantly. Just dont overdo it - its not necessary. Running 1 km every two days will already do.
Take everything with humour. That will also make you very popular with other people.
Eat well. When I feel unhealthy, I cook a soup everyday that contains good vegetables like tomatoes, potatoes, garlic, and olive oil.
Don't do things that you don't want to just because everybody tells you they are good for you.
Find something that gets you hooked, like learning how to program or building aircraft models.
If you want supplements, take omega acid of very high EPA/DHA rating. Their effectiveness is disputed, but I found them to increase my energy and well being, without any side effects.
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