I've had this feeling before and really, the only thing to do is to urinate and then be comfortable knowing for a little while, there is nothing left to wet yourself with. Probably not the answer you were hoping for. It's something that goes away if you don't dwell on it. Or at least it went away for me.
Adding a bit of humor to this, the first time I used any psychedelics, I ate about 4-5g of mush and had a terrible day at a crowded Long Island beach. The weather was brutal, you know that hazy, hot and humid that just ruins everything. Eventually, the heat and the crowds made us flee to our car. Being on Long Island, where else would we go but the mall with air conditioning. Remarkably, it was less crowded. For the record, yes, my friend operated a motor vehicle. Looking back, bad idea but we were young and dumb and luckily it worked out alright. Anyway, we had to really psyche ourselves up to get out of the car and go inside. And at this point in time, sitting in the hot car, I had the most realistic sensation of pissing my trunks. And I exclaimed to my best friend (you'll see why I say best in a moment) that I thought I was currently urinating. I promptly lifted my ass off the hot seat and my friend reached over to check the seat for me to be sure and after a bit of groping and patting, he replied "and I'm pissing from my fingertips". There was no incontinence of any kind.
In summation, try not to think about it and use the empty often approach.
