You're feeling down and nervous because of all the things you mentioned put together.
It could be that one big thing that you're unhappy about is making you unhappy about everything else; like a domino effect. My guess is that you're over-anxious about your exam results and are worrying about it because you feel that you've not done good enough (because you think you didn't do enough revision) and/or because your holiday was cancelled but you really wanted to go.
I find that if you're really really sad about one thing, it makes everything else hell. When really, everything else is just normal, but because your moral is bad at that moment in time, everything else seems bad.
If you look at it this way, if you were in a 'normal' every day mood, you wouldn't be worrying let alone thinking about if you've done a decent thing in this world worthy to be remembered, or you wouldn't overly-stress about which college you'll decide to go to, or that you've not completed my signature. Sure, you will think about those things, but they will by no means make you feel down on a normal day.
Obviously because we don't know each other, I don't know what's bothering you and why. I can only guess, and my guess is that you worrying about your exam results and the cancellation of your holiday combined is making you feel bad about everything else.
Don't you feel that way sometimes when you feel that nothing is going right for you some days? Some days nothing does actually go right for you, but in my opinion, most of the time a person feels that nothing is going right for them is because one major thing has upset them, so this effects everything else in their day.
It's normal to feel nervous about your results and it's normal feel unhappy about not being able to go on holiday after it has been planned already and everything. There's not much you can do really other than wait for your results and try and make yourself feel better about the holiday by doing other fun stuff! You can almost make up for it by doing other cool things. And honestly, you really don't have to worry about my sig!
Relax, and do things that make you happy.
Smile and the world smiles with you? Absolutely.