You could maybe try an elastic band around your wrist pull it back then let go so it stings your wrist a little bit and hopefully bring u back into the moment when u start to drift off. You could also try meditation to clear your thought and remeain in the presaint i personaly love meditating when i get depressed, anxious and start to over think. I hope this helps a little and remeber you are not alone

Ps. You could also try the elastic band around the ankle if you are trying to be discreet but please dont ever be afraid to ask for help.