You probably sound amateurish because you aren't very experienced yet. Take all the advice you're given here - but also allow yourself to be a beginner! Even Shostakovitch didn't start out as a polished musician. After all, if you were given a book on carpentry and told to follow the instructions, you wouldn't expect to come out with a perfect inlaid mahogany and rosewood gale table on castors the first time.
Cheer up. You'll get better as you go along. Keep one tape a month, and after a few months play them all - you'll be surprised how much better you've got.