Hi- I've suffered loads from UTIs for years- seems to me they come in waves like this that can be quite hard to shift. The reason lemon barley water is advised is for the barley- which I think is supposed to coat your waterworks and ease soreness. However, you shouldn't really drink anything citrus when you actually have an infection as this can aggrevate it, so I'd stick to the lemon barley water as more of a preventative than a cure (and same for cranberry juice). While you actually have the infection I'd cut out any caffeine whatsoever, and fruit juices. You can drink malt barley extract (a spoonful in a mug of boiling water/ milk) or boil up barley itself and drink the water- this can be quite soothing.
You can also buy cranberry extract to take as drops, and tablets.
My 'normal' trigger before pregnancy was intercourse- I was advised to empty my bladder straight afterwards, which can help prevent an infection.
Another is not emptying your bladder fully- i.e. go for a pee, stay there a minute longer & try again. Since I've been pregnant, especially as the bump increases in size, I've noticed that I can't always tell if I've emptied my bladder completely, or sometimes I feel like I'm dying to go, but only a very small amount comes out. So as well as drinking plenty, I'd make sure you go to the loo every hour or two, whether or not you feel you need to or not. And try never to hold it in.
Also, lastly, different antibiotics are sensitive to different infections, so your GP should have tested your urine sample at the lab and not just with a dip stick in the surgery to prescribe the most appropriate antibiotic. It may be that you need to try a different type.
Good luck & I hope you feel better soon. X