Don't put a value on beauty?
I know this is pretty impossible to ask - and I don't blame anyone who struggles with this. Society showers praise on to beautiful young girls, so of course they can't help but internalize some of that as their own self-worth. And then of course it feels like a significant loss when your beauty fades and the self-worth decreases.
Best thing is just to never let the superficial praise/kindness/affection that you get for being 'beautiful' impact your own definition of yourself.
If "you" is defined by your kindness, intelligence, bonds, friendships, family, successes, etc. etc. then being old doesn't hurt at all. In fact, being old tends to make all of those things better.
So reject the trick and the notion that your beauty = your worth. and build your self on much stronger foundations that will improve with time, and not diminish.
In terms of the joys of being beautiful, that is something just to chalk down to the past. You can only have one youth, you enjoyed it, don't grieve for it. We can't go back and be teenagers again, have first kisses, leave home, be young and free, so there is no point trying to go back. in stead just go forward to the good parts about being old. Nothing will make you more sad than being 'older' and trying to be young, because every time you try it will just highlight how not-young you are. Nothing will make you feel more old, than being surrounded by young people etc. Embrace the good parts about being older. You are likely wiser, calmer, richer, more capable, hopefully have a family/relationships/friendships/ambitions etc.