Rest and sleep and time. Without stimulants (not just 5-hour and caffeine, but also added sugar) to keep you artificially high, your body is starting to feel the real fatigue these things have kept from you all these years. And the only real cure is to give yourself what you really need: the time to recuperate.
Short term: try coffee again, or tea. Or (and this is sort of contradictory), catnaps. Better to deliberately close your eyes for 15 minutes in your car or on your desk than let your tiredness catch up with you and force a timeout on you.
As you said, your energy will rebound (and then some!) in no time at all.
Hope this helps. Good luck!