Most of the people get nervous with the very thought of an interview. You might be having much better things to express, but you might be lacking the skills on how to present it in an impressive way. Learn the tips and tricks regarding how to dress, how to prepare your documents, the way you carry your body and how to move around during an interview.
For many organizations, certain people and communication skills are more important than expertise in the product or industry. The latter can often be taught, but the patience and empathy required to effectively help customers is much, much harder to learn on the job.