The last thing to find out is if there is anything physically wrong. What you describe can be caused by chemical imbalances in the brain. These can be restored by pills but you need a Doctor to find out both if anything is wrong and what to do about it.
It's very normal for teens to have ups and downs because their hormonal level is still balancing itself. If you are in control I'd try to wait it out and see if things get better as you move towards your twenties.
You've already found out that looking forward to the good days helps you go through the bad ones. Keep focused on that,

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Take it easy. There are exceptions to everything I say.