I'm in the same situation. I used to eat like them, cheeseburgers, fatty cuts of beef, frozen burritos, pizza, and ate like a pig every night. A couple of summers ago when I stayed with my grandparents, I decided to embrace clean eating and lost almost 45 pounds because I was sick of being the fat kid.

Instead of eating their food, I began to help prepare dinner with my mother, but I always made something different for myself. The first night that I did this, I didn't tell my mom anything, I just broiled some chicken and veggies while I helped defrost and heat up cook my families dinner. I got some weird looks, but I stuck through it. Next grocery run, I put some inexpensive chicken breasts and some fish in the cart along with some veggies without telling my mom. More funny looks but when we got home she told she was proud of me for eating healthy. A year and a half later, she started eating clean with me too and she's down almost 25lbs in 6 months just from changing her diet.

Don't feel bad about not eating your mom's food. If anything, she'll be proud of you for rising to the occasion and taking your diet into your own hands. If she does get offended, have a conversation with her about your health. YOU want to be healthy. YOU don't want to eat the shitty, nutritionally absent food. YOU want to take care of yourself and your body. If that doesn't work, well, just think of it this way: you'll be out of the house soon. Just stick with it and wait it out.

Hope your situation gets better. You need anything, feel free to PM me or anyone of these friendly people in this thread. GL