Hi, teeth clenching can be due to stress, anxiety or due to abnormal bite.
Next Steps
As the doctor said, it comes as a custom fit.. pre fabricated doesn’t fit well. Visit your nearby dentist and get it done. Also, meanwhile practise some stress reducing techniques like yoga, meditation or exercises to relax your jaw muscles. If your jaw pains in the morning, you can consult a physician and take a muscle relaxant for the same.