I understand exactly where u coming from. When I first started playing in church, i was so nervous my hands shook uncontrollably and I also messed up songs that I had practiced over and over. It's just stage fright, everyone experieces it from time to time, although it's real bad for some people.
The only way to get over the nervousness is to play in public more. Also, play with the nervousness, don't back down and shy away. If you stick with it, usually that nervous feeling will leave. You have to keep in mind who you're playing for, God. If you make a mistake, work with it, don't stop. Most non-musicians can't tell the difference anyway, so just keep going and it will get easier.
I also get nervous when I attempt to play a new song in church. I be wondering "Will my church like it?", and things like that. Sometimes it comes out good, sometimes I bomb, LOL.