Why can't I believe in myself?

For years I wonder why but I never really found the answer so far.

For some weird and mysterious reason I can't seem to stay confident about things.
2 weeks from now I have to do my second admission test to a film highschool and when I signed up for it in June it all looked okay and not too hard.
Or better said, I was sure I could do it, with ease.
Now I'm not so sure about that anymore ...
When I'm reading the requirements and the things we have to show I feel like just giving it up.
How hard can it be to make 5 pictures of my own environment and interest that all can relate to eachother for strangers ?! I'm probably being too picky or too perfectionistic but I really only get as far as 2 pictures.

I'm just getting scared, scared to be rejected again, scared that I'm forced to go and do something totally different, something I wouldn't be passionate about.

I don't want to make choices , not yet.
I feel too young , I should be living a sorrow free and happy life.
But instead of doing so I'm alway worrying, having insomnia because of it, barely go out the house and getting in a constant state of some sort of nervosity.

And if all the sorrow wasn't just enough so far, it turns out that the apple of my eye, my sweet big bear dog Casper has heart problems. I know he's 9 already and that it is quite the average age for a Newfoundland dog to get to but still if I lose him in the next month I wouldn't know what to do. He always is there for me when I need a hug even  although he can't hug back and he likes to pretend he doesn't like to be hugged, there's something in his gesture that shows he does, he can comfort me and calm me down when I need it.
Loads of people laugh if you say an animal can be a best friend but dogs are the most loyal and caring animals around.
Oh dear, I'm crying while writing this, I can't stand the thought of losing him :(