First — it is not the end of your year
A single test is one data point out of many. Your final mark is built from a whole year of assessments, and one low result is completely recoverable if you act on it instead of spiralling about it.
Do this within 48 hours
Get the paper back and actually read every comment your teacher wrote — most students shove a failed test in a bag and never look at it again, which throws away the most useful feedback you will get all term.
Sort your mistakes into two piles: "I knew this but rushed/misread" and "I genuinely did not understand this". These need completely different fixes.
For the rushed pile: redo those exact questions slowly, timing yourself, to prove to yourself it was a pace problem, not a knowledge gap.
For the genuine gaps: find that topic in a past paper, do 3 fresh examples, and ask your teacher one specific question about the part that broke down — not "please explain the whole chapter again".
Talk to your teacher, don't hide from them
Teachers respect a student who comes back after a bad mark with a specific question far more than one who goes quiet. It also means they can flag it early if there's a real problem, instead of finding out at the final exam.