My meth use has gone up and down over the years - down to almost none lately - but yeah, when you use a lot, it's like you get sort of permanently jacked up, and not in a fun way.
I agree about carbs. I never want to eat when I'm on meth - one of the attractions for me, be high and lose weight! But make yourself, really. It helps.
I have used benzos to sleep, but be careful you don't develop a parallel addiction there.
Just ignore the "you should just quit, man" crap. You cannot say the word meth around here without people losing their freakin minds. I think you can use responsibly, but it's a strong drug and you need to stay ahead of it. I agree 100% with Tytan, that if you wanna use something as powerful as meth, you have to do REALLY good self-care to balance it out.
Are you taking supplements? There's a ton of stuff here about supplements that's very helpful.
Are you getting a healthy amount of exercise? Seriously, man, a regular workout routine will help keep your mind and body more balanced, it really makes a huge difference for me.
If you want to do meth and not get into trouble, you have to have rules, and taking extra-extra-good care of your body in other ways has to be one of them. You're 18 now - I'm 41, and trust me, your body's ability to throw off the unpleasant side effects will never be greater than it is right now. If you want to grow up to be a healthy old speed freak, you need to create good habits now.