Dear Gary,
Indeed the drug which you are taking may lead to
decreased libido, as a symptom of drug induced depression. I suggest you stop using it as it may hurt you in a lot more ways. You can read a short review of the effects on wikipedia - http://en.wikipedia.org/wiki/Mephedrone#Effects
In order to feel better I would suggest first stop using the drug completely, try some sports, any kind of enjoyable physical exercise, sex, if you are able go on a vacation.