What can I do to stop being a mouth breather at night?

If it's a problem of your airways not being open, in addition to what was already recommended, I would consider using an essential oil blend called Breathe:
http://bit.ly/2A6Y9mc

Hi Dr. Eugene! One simple thing to try is to place paper medical tape over the mouth before going to bed at night. I learned this idea from Robin Rothenberg who is a yoga therapist who investigated the Buyteko breathing method and presented a very informative webinar about healthy respiration. I learned from the webinar how maintaining a certain level of CO2 actually helps support the process of oxygenation and mouth breathing is detrimental to regulating that level of CO2. Here's a link to the presentation if you're interested: https://essentialyogatherapy.com/events/2017/3/20/pranayama-redefined-breathing-less-to-live-more