seems to me like you have a self-image problem, not uncommon at all and you're actually probably in the majority of people your age, so take comfort in that. the best solution i've found (and yes i've struggled with the almost exact same problems) is to just find something you really dislike about yourself and work on it. don't take 5 huge goals and fail at every single one because you're stretching yourself too thin, that will only hurt you. you mention athleticism a bit so i feel as though you should find something athletic that you really enjoy or helps improve yourself.
get a gym membership, you meet a lot of cool people there, and trust me when i say noone at the gym truly judges because everyone started from the same place. I used to be deathly afraid of the gym simply because of social anxiety but i forced myself to go 3 times a week and just run as long as i could on the treadmill. i found that just the exhaustion after really pushing myself gave me a large boost of self-confidence. so at least try going and see if it's rewarding to you.
another option is to find something athletic that isn't technically a sport. for me skating is the best workout i can imagine but yet it's really not a contact sport or really a sport at all. it's just a fun social thing that you can go to a local park and meet new people and work on a skill you haven't had before. not saying go skate, just saying don't keep athletics to such a small field, think big.
and really i guess the biggest thing is simply explore everything. keep your eyes open and when you find something you like, loving do it. don't let anybody judge you and definitely don't judge yourself. your self image will not fix itself, but you're in total control of fixing it. good luck to you and i hope it doesnt take as long for you to deal with these problems as it did for me.
