I've got a medicine spoon I use as an STP at home, and usually I use it when I'm just out of the shower or something, so I'm not wearing anything, and it's easy then. But I've found that when I try to use it while wearing underwear and especially pants, it's really hard for me to make myself go. I guess it's because I've been trained to not pee until my pants and underwear are off my whole life.

So do you guys know how to defeat this barrier? Is it just by doing it over and over again until it comes easily? Or is there like a trick to it?

I know exactly what you're talking about, and yes, you do get over it just by forcing yourself to do it over and over again. At least, if there are any tricks I didn't know of any. Keep trying. Once you do it the first time it gets exponentially easier and easier.

I am in a very similar situation, fine in the shower.  With my jeans up it is hard to get started which takes lots of practice, I think, and needing to go bad enough.  I also have problem with keeping a good seal with my parts and getting the end low enough so the pee will flow out.  The zipper on my jean doesn't go low enough and I don't want to have loose the waist to lower my jeans.  This only makes getting started harder since I am afraid I could start leaking.

I don't know if you are nervous and holding it tighter. But a lot of STPs require you to hold them loosely, basically that would be anything with a funnel arrangement.

--Jay