Why is it that i can sit on the toilet for an hour and keep peeing on and off through that whole time?

i feel i can't go out til i have completely emptied my bladder and to do so can take up to an hour! its like short bursts of pee, but then i feel the pressure to go and its the same again, kind of gushes. i don't over drink water, its been going on for about 2 years. no urine infection. but i am diabetic. its embarassing and i have to know where the toilets are if i go out as although sometimes i can last up to 4 hours, sometimes it can be twenty minutes!grateful for any ideas. gp just said to cut back on what i drink, that i can't pee more than i drink, but i am sure i do!