According to the symptoms you describe (diarrhea, shakiness, apathy, more anxiety, depression, no appetite, sex drive, no emotion) and data about decreasing the Methadone dosage... you could be experiencing a withdrawal crises combined with anxious-depressive disorder. Methadone causes withdraw syndrome symptoms that are similar to those experienced on heroin.

Treating addiction from heavy drugs like heroin can very complicated. The most important thing in treating heroin abuse is not decreasing a methadone dose but preventing a relapse to heroin use. Changing the addiction from heroin to methadone (methadone program) is the first step in many treatment programs. A methadone treatment program can last for different durations of time depending upon the individual. A decrease in the dose of methadone is not as simple as you think. In many cases, a methadone treatment program can last for life.... but some feel that methadone is better than heroin abuse b/c people on a methadone program can live fairly normally.

You must consult a specialist for treating drug abuse as you decrease the dosage of methadone.
There are physical withdrawal symptoms that can be monitored by a treatment specialist. You can also attend 12 step meetings like Narcotics Anonymous, or Alcoholics Anonymous to have a support group who understands what you're going through, and to learn new strategies for a life without drugs.