Relax. Stressing out and getting tense won’t help.
You’re a Junior taking well advanced classes. That’s to your credit.
Your friends are lying.
Are you focused on learning stuff or getting a grade? Focus on learning stuff and focus on the things that are giving you trouble. Don’t waste time on the stuff you know. It’s math. It builds. Start at the place you get stuck/trip up and work out what’s giving you trouble. And get help. If you’re trying to gut it out on your own, you’ll never get there.
Breathe and inhale positivity. Stop worrying about getting into selective schools and learn the advanced math and physics you signed up for. That’s what’s important.
You’ll be fine.