Becoming muscular and clear-skinned isn't going to make someone suddenly like you romantically, unless you want to admit she's incredibly superficial.
Doing nice things for her also isn't going to automatically do it either. Again, girls aren't vending machines that will pop out a relationship or put out if you give them affection or gifts or "be hot."
My husband is in his 30s and still gets acne at times and is super-skinny. Does that matter to me? No. I find other physical aspects of him attractive, and emotional aspects of him super-attractive.
Do I wish my husband looked different? Sometimes, but it's mostly because his posture is so bad, that I'm worried about how it's going to get worse when he gets older.
I think you do know what love feels like when you're in high school, but in my experience, someone that young has a skewed perspective of how love
works.
When I was younger, I thought no one would love me because I wasn't pretty. I had no boobs. I wasn't popular. I wasn't even social.
None of that matters in love. Sure, being all of those things makes it easier to date, but when you find the right person, you find the right person.
Even if you love someone, it doesn't mean they're going to feel the same way towards you. It's painful to acknowledge that a relationship isn't going to happen, but if someone doesn't like you that way, there's nothing you can do to make them change their minds. You can either wait for a what-if that may never happen, always changing yourself to try and please this one person, or move on.