Tell your mom? I agree with the other posters—don’t. She just won’t understand, LOL.
Now, if you’re 18, and your mom won’t leave you alone long enough to do the newbie routine in private, that’s a larger issue you need to deal with. She’s just got to understand that you need some space and time alone—not just for PE, but for other stuff. She shouldn’t be barging in on you. I’m about your age myself, and I know it can be tough to get this point across to your mom, but it’s totally worth arguing and griping to get some secure space of your own.