Well, it might be helpful to take a break for a week or two to allow yourself to recover and think about the entire experience (preparation, strategy, materials used, test day, etc.). I would also encourage you to expand the information in your first post. Mention as much as possible. This might attract additional members to comment and, perhaps, offer solid advice. It will also help you find closure and perhaps understand more clearly what happened. Once you feel ready for your second attempt, you will develop a better strategy, be more committed and confident, and the nerves will not be there anymore.
While losing some confidence in such a situation is perfectly normal, it's part of a process. You're trying to find some answers to make sense of it all. Notice, that I used the word
process. Pretty soon, you will know exactly what went wrong (it sounds like you already reached this stage) and will improve your performance based on that information. Again, just take some time off and keep thinking about your GMAT journey. Search for relevant topics on this website and keep asking questions!
Good luck!