Hey, I'm not really good at giving life changing advice but I did want to say a few things. Basically, I'm in the same place as you, i'm turning 19 within the next few days, I did 3 years of a level (which has been the most LONGEST thing EVER), and even though I did spend a significant time revising and following revision tips, I didn't get the best grades possible. Yeah, I am feeling down about it and the thought of being a failure has circled round in my mind a countless number of times, especially as I am a year behind my actual year group. However, having that mindset just brings us down into a darker place where we feel like there is literally no hope.
OP, we have genuinely come to far to give up and we also have just started out with our life. There are endless opportunities to be taken up and tbh grades aren't the be all and end all. Yeah, true, it does give the opportunity for a easier access into higher education, however there are other routes that can be taken such as foundation courses or apprenticeships, or literally just taking time out to travel and gain some life experience. In regards to guys in our year group, majority of them are trash and aren't ready to settle down or be in a relationship, so be wary of them and keep them at an arms length while you get yourself grounded.
Speak to your mum when you can though, and when you're feeling up to it just take a walk around the block or something, that helps to clear my mind when i'm annoyed...or play music...literally whatever is your way to outlet stress, do that.
But yeah, don't give up just yet. Everyone has the potential for success within them, you just gotta find it out for yourself. I'm still figuring it out, but it'll be okay 😊🤞👌
...sorry for it being so long >.<