Hey, I play mandolin too! I feel for you, sweetheart, as I have also experienced nervousness playing music with others, as opposed to just playing by myself. It is a bit like public speaking, but playing with a group has its own particular challenges. I find the other instruments tend to naturally drown out the soft-spoken mandolin (especially them #$@&* banjos),and some musicians just love to "show off" and compete. Intantaneous Personal Magnetism is a good paraliminal to use, as there is more to the musical-group experience than just technique. You can also look into You Deserve It to help with some your other blockages.
Debbie