Why this gets harder to manage as a diabetic

A prostate that's grown can press on the urethra, which is what narrows the stream and makes it hard to empty properly. That part happens to plenty of men as they age.

What's different for you is that high blood sugar over time can also affect the nerves controlling the bladder, so the bladder isn't emptying as forcefully as it should either. One problem pushes on the pipe, the other weakens the pump.

That combination is exactly why the usual advice, cut back on fluids in the evening, doesn't get you very far.