Evey, all opioids work by interfering with your endocrine system, causing it to manufacture less of your natural pain-relieving hormones (endorphins); and reducing your daily intake by 50% is going to affect you. Continual false fatigue is a symptom of this.
What you need is some gentle exercise to try to stimulate endorphin production again. A good long walk in the country would be ideal (and the fresh air won't hurt, either). And reward yourself with some chocolate on your return.
An experiment you can try (with the help of a friend who is a DJ) that might help you understand a bit better: Get a direct-drive turntable (it won't work with any shitty belt-drive toys). Select 33 rpm., remove the mat and spin the platter with your finger until the appropriate row of strobe markings -- 33/50, in the UK -- are steady (meaning you have reached 33 rpm). Start the motor while still turning with your finger, and maintain this for at least another 10 turns. Now take your finger away, while keeping watching the stroboscope markings. They will appear to run backwards, meaning the turntable has slowed down, then return to normal as the motor catches up.
EXPLANATION: The direct-drive turntable is a feedback system. The motor continually adjusts the amount of energy it is supplying to the turntable, so as to maintain a constant speed (there is a sensor embedded in the motor which tells the control electronics how fast it is going). While you are supplying some of that energy with your finger, the motor has less work to do. As soon as you take away that external energy source, the turntable slows down and the motor must now work harder to reach its operating speed. It's a lovely demonstration to watch. [Says the qualified engineer .....]
Now think of the turntable with the motor and its control electronics trying to maintain the correct speed as being like your endocrine system trying to maintain the correct balance of hormones in your body; the motor represents your glands, and your finger represents opioid medication. Reduce your intake, and your hormones will be out of balance until your body can catch up with "normal" under its own steam, as it were.