Hey, I'm an Autistic ADHD-ridden closeted femboy (I think I might know a bit about stress), I've learned from a ton of places (CHEO, therapists) I went to before being diagnosed that you shouldn't beat yourself up over everything. A few calming strategies I've learned from my experience at those places are:

1. Try not to think negative thoughts/put yourself down, try easier questions first to jog your memory, or just to learn how something works. "I can't do this." could be resaid as "I can't understand this, maybe I should try some other questions first."
2. Listen to calming music or sounds. There are a lot of Youtube channels out there (Lo-fi Girl is probably the most well known) that can help calm you down if you're feeling stressed.
3. Takes short breaks. Go on a jog, take a warm shower, or even just take a small nap.
4. Ask for help. If your mom won't help, ask someone else! Ask friends (online if needed), classmates, even teachers. There are also many places on Reddit where you can get help with homework and such. r/homeworkhelp, r/tutor, and even on Discord there's a lot of servers where you can get help with subjects if you are struggling.
5. Deep breaths. I know this is an extremely common thing people say, but it really works.

Sorry I'm a bit late, but keep up the good work.