How do I stop grinding my teeth?

My dentist says I grind my teeth at night (she can see the wear on my back molars, eek). I know my mom does this too, so it’s probably kind of a genetic thing. But I am also under a fair bit of stress from being in school. My dentist said to get a mouth guard and I tried that but always end up spitting it out in the middle of the night. Any other suggestions? I don’t want to grind my teeth down to nubs!!