It is interesting how some people have such difficulty with wetting their diaper even if they want to, and others don’t. I have always been able to let go in diapers or my pants when I want to wet no matter where I am or what position I am in, and I’ve never had a problem peeing outside or other pee shyness. Maybe it’s because I never had to get my body to “unlearn” years of potty training. As long as I can remember, I’ve enjoyed wetting my pants, and I started wearing and using diapers I took from my younger brother at age 4, so I’ve pretty much been doing it as long as I was “officially” out of diapers.
My wife, on the other hand, can’t let go at will and will get to the point of extreme desperation before she is able to pee anywhere other than a bathroom or at minimum a porta-pottie. When we are at a beach with no facilities, she has tried to go into the water to pee, but can’t relax and let herself go until she’s about to have an accident, and even then it is with difficulty. She doesn’t join me in wearing diapers, so I don’t know if she would have the same difficulty letting go in one, but I imagine she would.
As for how to teach your body go in a diaper, like others said, it comes down to doing it often enough so it is second nature. I would recommend starting in places and at times that are “safe” from being discovered and where you aren’t worried about potential leaks or similar issues. Being at home alone is ideal, but the privacy of a bedroom or even bathroom will make you feel safer than in common spaces with family or roommates. In the morning, put on a diaper and try to wet it just before getting in the shower - you will need to go, and there is no worry about leaks or cleanup as you are showering anyway. If you can’t easily put on a diaper, aren’t able to wear often enough to get used to wetting it, or just having a diaper on makes it mentally difficult to go, try wetting underwear standing in the shower or tub instead. Just let go before stepping under the water. You’ll get used to letting go when you aren’t supposed to, and you can immediately wash off and rinse out the undies without anyone suspecting. If that is too difficult, you can start with simply peeing in the shower, then work up to underwear/diaper in the shower/tub, eventually to diaper under clothes outside of the bathroom. If worries about leaking are making it difficult, find a time you can push your diaper's limits and learn what it feels like up to the point of leaking. Just before doing laundry is a good time for this as you can easily wash any wet towels or clothes, and no one is the wiser.
