Breast milk

I believe I messed up my milk supply I was pumping 3 to 4 times a day getting 5 ounces out of each breast in one sitting. Yesterday I couldn't get but an ounce and a half out of each breast after 20 minutes of pumping. Now I have to pump every 2 hours just to get 3 ounces each time. Sometimes it takes a full hour to get that much milk. Does anybody know anything I can do to make my milk abundant again?