UTI that doesn't go away

I've had a persistent UTI that gets a bit better with antibiotics and as soon as the course is finished it comes back. They've already determined which antibiotics this particular strain is sensitive to, and I've tried all of these in sequence, but nothing has worked. I've now been told to take trimethoprim for an additional week (after completing a 1-week course of that antibiotic a few days ago) in case it just needs a longer course. I'm really worried it won't work though because I haven't felt like it made a difference during the first week I took it. This situation is really stressing me out and I was wondering if anyone else had to deal with something like this and might have any advice. I'm 28 weeks pregnant at the moment so still have a long way to go.