hey saiyansteph, welcome.
i'm sorry to hear all of this.
it's ok to worry about clothes not fitting, some clothes are made differently anyway. wear whatever you feel comfortable in i say, and what you like the look and feel of.
i'm sure you do look good in whatever you do decide to wear, and i'm sure you are pretty. you are good enough.
it's ok if you show some skin such as your belly if you feel comfortable. i'm sure your arms aren't flabby, but if we have a bit of extra weight than others, or less, that's perfectly ok. someone's appearance doesn't define them, it's what inside that counts, that's what i live by.
you're not ugly at all, i'm sure you're pretty! of course i mean all of this in a friendly, reassuring way (i'm female also). people do care, and they won't be lying when they call you beautiful, i know i'm not, and others here aren't. i'm sure they genuinely mean it, i do.
i hope the wedding goes well, i'm sure you'll look wonderful. hugs, if you want/like them.
i think being a young person, especially a female, with society, etc. makes it hard too. you're not alone. it's ok to not be comfortable in our own bodies. it's what's inside that counts in my eyes.