get hold of a book called 'the inner game of music' by barry green / w. timothy gallwey. it sounds to me that your performance problems are psychological, and you have to change your way of thinking.. so read this book and hopefully it will be a big help. good luck!and why not just play with the score? theres no rules against it is there?
First study the music in all aspects. Then practice correctly:then learn how to concentrate:.