I am afraid it is highly likely you do have a UTI, that normal NHS testing does not pick up.......it's highly likely a bladder wall infection hidden under a bio-film......and the way you are being given the antibs will only be making it worse.....encouraging the bugs to dig deeper into the bladder to form an even stronger resistance against the antibs.......
If you can afford private I suggest you see either Professor James -Malone Lee ( London ) , or Dr Anna Catriona ( Lincolnshire ) , both specilists prescribe " very high" antibs to blast these resistant embedded bugs, and the antibs are changed as necessary as an embedded infection will very likely have more than one bug, so as one antib kills off one the next set of bugs rises to the surface and so on until they are all eventually eradicated.......this treatment is not for everyone, it all depends if you can live with you're symptoms.........
Dr Anna is a more whole person approach as will look into HRT, alternative also.......all bladders contain at least 15 bugs and the idea is the good and bad live in harmony, but when the badger too big for there boots then they need a nuclear bomb to blast them.
I am on a closed FB group,where there are ladies who lives where intolerable and told they had IC, and are doing extremely well, it was infection all along......the term IC means " we don't have a blinking clue".