I feel your pain Nathan. I used to have the exact same problem. The thing is, nobody will ever sing your songs like you sing them, and for singer/ songwriters, a song can't always be separated from their own performance of it (I think this is true of my songs). It took me a lot of years, a lot of practice, and a lot of very nervous performances to get over my stage fright. I still feel nervous whenever I perform, but I have enough control over my voice now to not let it get in the way.
Keep at it, Nathan, you can overcome it. It can just take time.