Fixing my life as a senior in high school

Hello. This is my first time posting on Reddit, so please excuse me if this post is in the wrong subreddit/too long/too personal etc. I am looking for some advice on how to improve my life. I am 16F, in my senior year of high school, and I am realizing that I'm going to university next year and I am extremely unstable in terms of my coping mechanisms and reactions. There is no way I will make a suitable adult.

I was a "gifted kid" for all of my life but in my senior years of high school my grades have started to drop below my peers. I used to be averaging high 90s in all my classes and now most of my classes, especially harder subjects like Biology, Chemistry, Calculus etc. are all becoming dangerously lower. I am terrified that with marks in the mid-high 80s, especially considering the current grade inflation, that I will not be accepted into any universities I apply to. Which would be a massive failure on my part.
I also have zero self-esteem. I can get a bad grade on a test and lose my confidence in the subject for the rest of the year. If I have a bad interaction with a friend I will automatically assume they now dislike me. I began performing poorly in my sport and quit. If I have a bad day at work my brain convinces me that I now hate my job.

My extremely low self-esteem and lack of confidence in myself has also translated to my appearance. Many people say I am a conventionally attractive, but I cannot bring myself to believe them. I have spent too much time looking around my face that now I hate it and avoid mirrors and pictures whenever I can. I also used to be extremely lean and in perfect shape but with puberty/depression/health issues I have now gained a lot of weight and can no longer find a single thing I like about my body.

All of these cumulative things have taken a toll on my mental health and I have developed very destructive habits (will not go into more detail but you can probably guess) to distract myself.

I realize that these are all privileged things to be complaining about and all I have to deal with are first world problems. I am sorry if it sounds like I am complaining. I would just really like some advice on how to grow to be more confident in myself and more resilient so I do not have to resort to harmful and self-destructive coping mechanisms that perpetuate the cycle. Thank you.