Why yes it is difficult to urinate while having a hard on. When the penis is erect the tube from the bladder is essentially pinched off. Something that I was taught years ago may have you. When I was close to cumming, a woman showed me the pinch technique to help keep me from orgasming. It would soften my cock slightly, and give me more fucking time. You could use it to reduce your erection. Just form a circle with your thumb and index finger, and squeeze your cock at the base. It should allow your cock to soften and then you can pee. Works for me.
I have even swallowed pee from both the old gf, and the current gf. Old gf would suck and drink my pee too. She got me very excited and turned on with pee play. We still chat, but she lives far away now. I will keep playing with the current gf, and maybe sometime she will be even more turned on herself! Drink lots of water too. Keep playing if it is a turn on too