Have encouraged therapy but was not successful. Therapist was on zoom and not good .
Sometimes it can take a while to find a good therapist.
Have you explained to her that you going away with your DH is normal adult behaviour for coupes and if she was in a relationship you would expect her, after a while, to probably do the same? She probably doesn't realise how much this is affecting you.
If she is saying it's anxiety, what has she done recently to try and ease her anxiety? Has she been to the GP and talked about trying medication? Has she referred herself for Talking Therapies or is she relying on you to just never go away until she says you can, which might be never if you're all stuck in this limbo with her not getting help and you not going away for fear of upsetting her.
When you say "there's something missing mentally" do you think that instead if this, she ciukd actually be ND? I know it's often trotted out in MN but she is showing two classic signs, anxiety and an over reliance on you.
I woukd talk to her over the weekend if you can, my DD always responds better if we're on neutral ground like a coffee shop. Acknowledge her anxiety and show understanding for Joe it must affect her but ask her what she's going to do about it as you don't like seeing like this. There are so many positive things that you she can do to help herself these days.
I'd read up on ASD in young women and see if any of it sounds familiar to you. If so, think about brining up the possibility of that with her too.