As a woman, I know hearing after years of doing it, that I was bad at oral sex would be upsetting.
How long have you been together? Why did you let it go on so long?
Too dry is not a problem you hear too often. Before you bring up the sensitive subject, are you 100% sure she doesn't have dry mouth issues? It may sound silly, but it's an actual medical problem.
If that's not the case, and she's already not being too receptive to your constructive criticism..... try this...
Do a little googling on blow job techniques. (Sounds silly, I know) and find an article about the correlation between more saliva and better BJs. Once you find one you like, show her!
"Hey babe, I was reading ______. It says a blow job is more intense if you use more saliva. A lot more apperantly. It sounds really hot, can we try this?"
Whilst getting head, tell her what you like, or don't like. You can coach her a bit. It won't be as awkward because the two of you are trying something "new".
I hope that helps.
Thank you for your kind and thoughtful help.