Some vets, like doctors, tend to only treat symptoms and not the cause, because its easier and you get "immediate" results. You are 100% NOT TO BLAME. You aren't a vet, how could you have known?! Thankfully it seems like your vet now is a little more knowledgeable, although even I'm not a doctor but after six UA's and no sign of bacteria or crystals every time, after the SECOND UA came back negative (occasionally you will have a mild UTI that just doesn't show on a UA so you will treat with antibiotics to see if it helps) I would investigate other causes. Six times with no bacteria/crystals... clearly something else was going on. Shame your previous vet didn't seem to want to dig any deeper to find other causes... but glad your current vet does!