you’ll always feel like you did worse in an exam than you actually did! our brains tend to panic about the worst possible outcome and all the things we got wrong that we’ll be marked down for, but exams are marked positively so you don’t “lose” marks per se!
however if you do really think you haven’t gotten the grades you need, you always have other options. at lots of colleges etc. you can resit subjects if needed, and there’s nothing wrong with going to a college/sixth form with slightly lower grade requirements! i’ve done my a levels at a college and i’m still achieving great results with a place at a top uni, and the only grade requirements for my subjects was a 5 in 5 GCSEs (i had to get slightly higher than this to take a level english, but nonetheless). and if you don’t want to do a levels, you can always look at other ways to get yourself into a career you want — sometimes you can do this alongside retaking GCSEs!