itchy urethra makes me feel like i need to pee all the time

[brankulo2]

this started out of the blue. itchy sensation in the urethra of my penis that feels like i need to pee. when i pee, i dont have any pain, blood or discharge, but when done peeing, it feels like i still need to go. the more i try to go the more the itchiness seems to intensify. if i dont go, it usually goes away or decreases after 1-2 hours. at this point, when i start feeling i need to go, i know this is real and that i really need to go. but after do, it all starts again, the itchiness is back and it runs in this circle. it tends to get better a night, i dont wake up needing to go, in the morning, i wake up feeling normal, no itching, but as soon as i go again, it starts itching.  also, it sometimes feels like there is liquid stuck in urethra, at the tip of my penis, but there is nothing. the itchiness is present within last inch of urethra most of the time. also the itchiness seems to go away when i have erection, but after erection is gone, it comes back.

A quick background: I am 40 and have been a monogamous marriage for over 10 years.  I cannot imagine how this could be an STD of any sort since I have not been with anybody else for over 13+ years (ie...The web instantly leads you in that direction). i went to a doctor several times, they checked my urine, did psa test, did post void ultrasound and said everything is normal. doctor prescribed some oxybutynin which after doing research, i  found out is for overactive bladder, but i dont have any of the symptoms that i found listed for this condition. anyway, i was desperate so i said to doctor i will give it a try but there is no improvement whatsoever after taking pills for over 2 weeks.
any idea?

thanks