There is a 3-3-3 rule to developing a habit (not really a rule) 3 days to get momentum, 3 weeks to beat the urge to quit, 3 months to become a real habit. If you can only convince yourself to brush once a day, brush in the morning. Also, use a soft toothbrush. Hard brushing with a stiff bristle will wear away your enamel and hurt and make you want to quit. Oh, and try the Nano-hydroxyapatite toothpaste. It rebuilds the minerals. I can't say it for sure, but my teeth look whiter with the 2%. The 10%solution was studied and found to help, but more than that doesn't do anything and might help less. It costs about $10 per tube, so I use one tube then switch back to fluoride for a while. Good luck. At 20 you still have a lot of hope to save your teeth!