You have to build it down in steps. For example, if you drink one glass when you wake up, try to only drink half a glass. Drink a bit lesser of those things every day. If you build it down slowly, your body won’t even notice. After a while, you might notice something and feel the need to drink more, but you can overcome this. They say it takes 3 weeks to make something a habit. So if you can hold it out for 3 weeks, it’ll be easier.
You can also try to reward yourself. For example, if you manage to drink one glass lesser every day for an entire week, buy something nice for yourself or allow yourself to do something fun. Little rewards may make it a bit easier to hold on. Good luck.