Try this:
1. Hydrate well until you need to use the bathroom. Then go use the bathroom like normal. (I know, sounds crazy.)
2. When you use the bathroom, sit there when you think your done, try to keep leaking. Get used to this feeling of leaving the valve open. New urine comes into your bladder and immediately leaves. No stopping in the bladder.
3. Hurry and put on a diaper before you piss everywhere. Focus on keeping the valve open. If you think the valve is closed, relax and push it open. It should start to come easier and easier.
A funny thing happens to a potty-trained bladder. When it’s full, it tends to “lock” and take more effort to open than a lightly loaded bladder, at least until it reaches the maximum fill line. I’m sure there is some better biological explanation for this, but that’s my experience.
Do keep in mind that extended use of this method could lead to bed wetting or even conscious enuresis, so moderate your play and maintain your urinary functions.