Practice. A lot of it is practice. Even being only 20, it took me a little while to let my body get relaxed enough to pee in a diaper.
For me, one of the soeed bumps was getting my body to accept that it was OK to do. Having the right mindset from the beginning is huge. You need to commit to wanting this to happen, and wanting to let yourself change.
When I really started getting back into wearing diapers, I was still young enough that I could fit into 4t-5t pull ups, and when I could squeeze 2 complete wettings into a Goodnites. At the time, they didn't feel like strange things to me. They felt oddly normal and comfortable, and I was able to really use them for their intended purpose. That helped me achieve the right mindset because it let me imagine being a baby, or at that age where you know that going in your diaper or pull up or Goodnites was what you knew, and you were OK with it at the time.
Try doing anything you can to make yourself feel a little more comfortable. Try maybe keeping your diaper a bit loose. Putting one on too tight could be part of the issue. You'd focus more on being uncomfortable as opposed to peeing. Try sitting on the toilet a few times, as if you were normally going to the bathroom, but while wearing a diaper. There's something about your body's positioning that makes somewhat of a difference. Finally, just relax yourself. Being all tensed and stressed about it isn't going to help at all. Clear your mind, get comfortable and relaxed, and let nature take its course.