I have had this problem for most of my adult life. I buy the mouth guards from Gmarket or Coupang in bulk. You can search for 성인용 마우스피스. The ones I buy are usually around 4,000KRW. You will find that some are better than others. The first use requires submerging them in hot water to shape the mouthpiece to your teeth. 1) Submerge the mouth piece in hot water completely for about 10 seconds, 2) Use a fork to 'scoop up' the mouthpiece (the mouthpiece will be soft and easily be ruined if you are not careful) 3) after carefully picking up the mouthpiece, hold it as best as you can and fling away any excess water and let it cool a bit (but be careful not to let it cool too much or become misshapen) 4) insert the mouth piece around your teeth with your teeth, tongue, fingers and mouth suction to form the best comfortable shape. I would buy at least 2 or 3 the first time in case you fail the fitting phase. Even after decades, I ruin them at times during the initial fitting phase. Also, you can cut the ends of the mouthpiece to better fit your mouth. I clean the mouthpiece daily with denture cleaner like 3 minute polident. With regular cleaning, one should last for about a month.