If an infection is being found each time then it does mean there are things you might try. It might be a bit old-fashioned now, but have you ever come across Angela Kilmartin and her advice on cystitis and thrush? She was an opera singer, it ended her career, and she ended up becoming a bit of an expert on it, writing quite a few books.
I used to have lots of problems and ended up going to see her, though the books were very helpful. There was lots of advice on weeing after sex (not your problem I suspect!) but also on having a thorough wash after a bowel motion as that's where the bugs are usually coming from. Her "method" was to soap backside thoroughly then rinse with a bottle of warm water as you sit back on the loo. That way, everything is washed backwards not forwards.
I wonder if there is a mix of things going on here? You might possibly be re-infecting yourself in some way (and that doesn't mean personal hygiene is at fault, but that you're susceptible and have to take extreme measures as described above). The bugs may well be resistant to 3 day antibiotic courses, as DG suggests. And you've got the problems of ageing/thinning/oestrogen-deficient skin and tissues which are more susceptible to infection.
The bicarb idea is good (though it tastes grim - I used to live on the stuff in my youth) because it keeps your wee at a pH which makes it less acid. That makes it more comfortable and harder for the bugs to grow.
Like DG I'd keep on with the vagifem - it should help make your bladder and urethral tissue less vulnerable to infection and paid. And only use soap on your bum - it's too harsh for your vulva, etc.
Good luck - really hope you get this sorted soon.
