I had this problem for 40 years, During one of the trainings I took in clinical hypnosis I volunteered to be used for a demostration and brought up this problem to be dealt with. In my case I had "programmed" into my brain, an internal dialoge saying "Who do you think you are, You can't do that" associated with playing a solo - an artifact of an unfortunate exchange with my mother when I was about 10 years old. (a long story) With a single, brief, say 20 minute intervention of a skilled hypnotherapist...problem solved and I now easily and with confidence perform solo passages as they occur both in rehersal and performance. I recommend that you do some research and locate a skilled clinical hypnotist or life coach. One who is trained in Neurolinguistic Programming NLP would be a good bet. Check local listings for clinical social workers or psychologists who may have this training. If you are in the DC area PM me and I can give you the name of the fellow who conducted the training. He is best I know in doing this kind of intervention.