Recurrent UTIs

Since Summer 2014 (when I was 17) I have been having continuous bladder infections which range from mild irritation which I can control with Cranberry juice and water to crippling pain which stops me from being able to carry out day to day activities and even painkillers don't always touch. Anything can cause it to worsen such as sex, not going to the toilet, not drinking enough, wearing tight clothes, etc. I have been to the doctors at my home and university surgeries and neither of them take my problem seriously. I am expected to produce urine samples every time I go and if there is no extreme infection they ignore it (there is always some kind of infection). I have asked for long term antibiotics as I am concerned now as sometimes I feel the pain in my kidneys and abdomen however they refuse to give me any as there is no sign of a bug in my urine. Antibiotics like trimethoprim no longer touch the infection and in some cases worsen it. I have asked for scans and more help in moving forward with treating it as it is killing me living with this. Does anyone anywhere have any advice on what I should do?