I sure can sympathise with you, after spending a year dealing with UTI's crystals and stones in two dachshunds.

When was the last time you had a urinalysis? And did they tell you what type of crystals? There are Struvite and Oxylate that the dog could have..they are treated differently, so if they havent been analysed you may want to ask. Did she have evidence of bladder stones on the ultrasound? If the dog is still making crystals she will likely still keep getting UTI's because the crystals irritate the bladder/urethra.

Have you gotten a second opinion from a different vet? Thats what finally worked for me. They were much more agressive in trying to diagnose what was going on and although my Barahona was a challenging case, a year later she is completely recovered (although I give her a pill at night as a safety measure). Sometimes a different vet, or even a specialist in canine internal medicine if you live near one, can give fresh insight that your current vet may have missed...