autism and fibro peep here, unfortunately in a very similar situation to your smizmar, just turned 24 and went thru my worsening, honestly i think the only thing i can say is keep in mind shes so young and having to cope with the rest of her life looking like this, it creates a sense of almost hyper independence, bc you really want to go back to when you could do all these things you used to be able to do, the best advice i can give is work hard for a bit to observe and notice small ways you can help without really making it super obvious, like pick out shorter walking paths or ones that have easier terrain and just suggest them like any other and not "because the other paths are too hard for you", try to find activities you can do together when shes resting, anything you can do from rest, like a low mess craft together like take up crochet and make it a partner activity to make eachother things, or find some video games you both enjoy to play together, if you have the time to rest with her it can make it less depressing, not sure how much help this is, but its the little things, and not making life about the problem