Failing is not a bad thing if you have tried your best, all it signals is the task was more than you could manage.
failing when you have not tried is not so good as you will never know for sure if you could have passed.
Teachers can tell those who have tried and failed from those who simply didn't try at all.
Unless you try you can't be sure you will fail, you could suprise yourself by passing.
I was in the lowest English group all the way through high school, i thought i was thick, was teased to h*** and back, but when i looked at the exam paper, it was about something i had experience of, and although i finished the paper last and gave myself a really hard time about not doing a good job, I gained the highest mark in the whole school!
you are only young, and still sorting out what you can and can't do, this will take a few more years yet, until then you will often doubt your ability, even Adults doubt their ability at times, it is normal, but just 'cos you doubt it doesn't mean you can't achieve it.