To be perfectly honest, and please don't read this as critique or anything, i find the whole discussion about it being selfish or not entirely bizarre.
It always makes it sound like the default has to be altruism. As if you only exist for those surrounding you.
All you can ever be is yourself, your own person. And every person has their limits and breaking points.
Decisions aren't a one way street.
You're not the only one who needs to conform to others decision. If you made yours, it's no one else's fault. It's just a decision that has been made.
I'll have to chime in with @cyanidekitty, don't force this decision upon yourself.
If your heart wavers, take the opportunity to do some reevaluation. Your pain is incredibly relatable, and please don't think i'm trying to diminish it in any way, but your situation is not inescapable.
There are countless people who completely crashed out during school who ended up leading great lifes.
School is fucked. Everywhere. It's a system built for a different age that never evolved, because the people in charge have a "we've always done it that way" attitude.
There are those that can conform and there are those that just check out. That doesn't make you any worse if you ask me.
You're facing a system that expects everyone to conform to a standard simply not meant for this day and age anymore.
Small anecdote:
I was a horrible student myself to the point that i eventually skipped a whole year because i just couldn't take it anymore.
Bully victim, "weird kid" and quite frankly, i just didn't see the point of it all.
And then vocational school happened years later. The dumb kid that didn't go to regular school now was an A-grade student.
What changed? Context.
I knew why i was learning all that stuff. The context of it being work related changed everything.
You can make the decision to end it all whenever you like. You have a whole lifetime's worth of time for that.
But if your heart wavers, maybe all you need is a change of perspective.
There's this old video of an Australian man talking about water bottle prices.
"In the store, a water bottle costs 1 buck, in the cinema, it costs 3 and on an airplane it costs 5. Maybe you're not worthless, maybe you just need a change of location"
Maybe you're not half as horrible as you think. Maybe it's your situation, your surroundings that make you feel less valuable.
Take your hesitation to do some introspection. The world sucks, have you seen it recently? You can't be worse than that!