Why are my teeth falling apart despite good brushing?

I just got my braces off two weeks ago and my teeth have become so horrible since then. I brushed my teeth 1-2 times a day (I know it's not that great) throughout my time with braces (3.5 years) and when I first got them off I had like huge white spots on the top of my teeth, a little discoloration, and a HUGE cavity on my front tooth. I brushed my teeth enough for this not to happen! For the past two weeks, I have brushed by teeth 3-4 times a day for 1-2 minutes and flossed constantly, and since then my teeth have gotten 10x worse. My cavity has gotten much worse, I have TWO new ones on my front teeth (one on top and one on bottom) and on the white spots from lack of fluoride it is legitimately orange and has bands of brown despite me brushing CONSTANTLY. On one of the cavities I literally watched as pieces of tooth came off when brushing (what???) I'm literally close to tears because my parents spent close to $10,000 on braces just to have my teeth be destroyed. What is happening! And my mom won't take me to a dentist because it is not covered under insurance (totally ridiculous but oh well can't persuade my mom). Any actual help or explanation is appreciated! Thanks :)