I had a lot of trouble at first when trying to use my diaper. My first few times I knew in my mind that I wouldn't get caught, my diaper would leak, and everything would be just fine. My body, however, has been conditioned for 21 years to use toilets. So when I'm trying to tell my body "Hey look, it's okay. You can just go now", my body responds with "Nah man, this isn't a toilet. I can't go here".
Some tricks I've been using to recondition myself is to go in the bathroom, pull down my pants so I can actually see my diaper, and turn on the water faucet just enough for it to make a trickling sound. Ten I just close my eyes and hone in on the sound until I'm ready!
Hope this helps!