Okay, there's a lot I like here, but I'm kind of pissed about the last throwaway line going back to the fat theme.
"You were able to make yourself not fat! You can do anything because you have determination!" is probably well-intentioned but the converse would seem to be "If you were still fat, it would show that you have no determination. Fat people are losers! You are a winner because you are no longer one." I feel like the author maybe didn't think that one through. Hopefully. I guess the point was that someone finally praised her for that particular bit of hard work, which is fine in principle, but...
The protagonist herself is cool though. That was one hell of an advanced-level passive-aggressive burn. "Oh no, I'm so grateful, you've taught me I should never trust a scumbag like you," except all pretty-sounding XD