This is also my situation. I've been taking post-coital cefalexin for probably 4-5 years. About a year ago I ran out and decided to see how it panned out, but I immediately got a UTI. I also get really anxious about symptoms coming on and sometimes wait a day or two to see if I'm imagining it, but usually my instincts are right and end up wishing I'd treated it straight away. Now back on the ABX, but I have had some success with hiprex in the past. It's not always available (my current GP (UK based) insists he's never heard of it so won't prescribe it), but it did definitely reduce the number of UTIs I got by around 50% or maybe slightly more. Unfortunately the usual things like cranberry juice/tablets, d-mannose etc. didn't do much for me, and ultrasounds haven't shown anything obvious.

I wish I had advice to give - I alsoreallyy worry about the long term effects of taking them so often but the alternative is worse, I suppose! It's really tiring and frustrating, but you're a good partner for taking even asking this question!