I know what you mean about suddenly waking up and your face has fallen - I’m 44 but feel I’ve aged about 10 years in as many months!
I started using a few bits from The Ordinary after reading all the threads on here about it. Not seeing any difference yet but just paying a bit of attention to my skin will probably help in the long run.
One thing that could help you is to put on a little weight - I know you said you’re naturally slim and active so that might not be easy, but as we age most of us look younger with a little padding under the skin. And men like women in all shapes and sizes - but at an 8 you have a little wiggle room, even for men who like slimmer women.
Your comparison to telling a larger woman that she looks fine, even if she doesn’t feel it, is spot on. But imagine that woman has been told by her ex that nobody will want her because she’s fat. That’s why she feels shit. The larger woman whose DP tells her she’s beautiful and he loves her curves and big boobs will be much happier in herself. The way others see us matters of course. But you can’t trust the opinion of your nasty ex. Please believe that.
However, regardless of how you change the outside, you will need to work on the inside before you start dating too. You must believe that you are the prize, that any guy is lucky to have you, or you will end up being played - especially if you’re dating online - it’s a bloody minefield, even for people with self-esteem!
By all means take care of your appearance, but also spend some time really looking at yourself as a person, what you have to offer, what YOU want from him and how you see your future. Once you’re more secure in yourself you will be able to find an equal and fulfilling relationship, not settle for the first man who proves your ex wrong!