I'm not a GP, but have they not suggested simply drinking more fluids?
You've used a bottle of water and a glass of juice in your examples of what you drank, and you complained in the OP that you have to drink the cystitis sachets at once instead of just sipping on it throughout the day (these are typically dissolved in a glass of water - which is a very small percentage of the recommended daily liquid intake). From this it sounds like you're just not drinking enough. Fluid input does not match urine output because much of the water we ingest is absorbed by the intestines, so if you're drinking as little as your posts have implied then of course you're not going to urinate much. Most of the liquid you are consuming is being used by the body to carry out essential functions. The most common cause for dark yellow or brown urine (which you indicated in your latest post you have experienced) is dehydration.
tl;dr - increase your fluid intake—the NHS recommends 6-8 UK cups (1.5L-2L) per day, but the US National Academies recommends 11.5 US cups (2.7L) per day for women and 15.5 US cups (3.7L) per day for men—and see if that makes a difference. If yes, great. If not, go back to your GP.