Absolutely everything esther has said, she always gives great advice!
Your point about not being able to make it better for her is key @Beenthere279; you can only support your DD through all the ups and downs from the sidelines (and not even that right now)?
I too have an older swimmer, turning 18 soon, and still hanging in there, even after multiple lock downs, and circa 1.5 stone weight gain in just over a year. And injuries, so many chronic injuries, shoulder and knees. But unfortunately, some of her own making, poor technique for example that do catch up and aggravate growing bodies. Breaststroke was also her key stroke, achieving top 5 regional rankings at 13/14 (after late competitive start at 11/12, well behind her peers). Now turned into a competent backstroker.
It really is not linear. And the body shapes, it takes all sorts, which is really great actually. And more than the swimming itself, are all the other value adds - friendships, camaraderie, multi- tasking, and general cliche of resilience. It really is not for everyone, and whenever the exit is for a swimmer, most look back to their swimming days with positive nostalgia. I wouldn't describe my DD as a natural, and there are days where I can see her pain, but she keeps going for reasons now beyond just the competitive side, and form is returning slowly. Good luck on the swim path, and to your DD!