How can I stop grinding my teeth before bed?

Relaxation or Botox. Practice some relaxation procedure as progressive muscle relaxation, guided imagery, self hypnosis, meditation. The"spilling over" of tension from the subconscious to the muscles may be reduced. Botox injections in masseter & temporalis muscles can reduce strength of the grinding without compromising speech write down a to-do list for tommorrow & any problems so rumination lessens & you can relax.
Wear a bite splint. Many people grind their teeth in the day time, before and during their bedtime and during stressful times in their life. It always helps to eliminate stress from one's life and that should always be the first step but a well made bite splint is a valuable tool that acts as a habit breaker and more importantly protects your teeth from damage caused by abnormal grinding. Consult your dentist.