While I sympathize with parents whose children abuse marijuana, it is the prohibition of Cannabis that has made marijuana into a "forbidden fruit," more attractive to rebellious young people than it otherwise would be. Every negative experience anyone has ever had with 'marijuana' has occurred in the context of the "drug war."
The vast majority of people who use Cannabis do so without tragedy. Your child has to take responsibility for his/her choices. If marijuana were not available then it is likely that people who abuse Cannabis would find another, more harmful substitute (i.e. alcohol, hard drugs). In fact I don't encourage recreational use of marijuana by young people.
Personally, I didn't even try smoking ganja until I had graduated from college, in the late '70s. Alcohol was my drug of choice in those days, as it is for many young people. Many of the problems and pressures that young people find impossible to deal with are caused by the drug war. Regardless of the effects marijuana has on people who use it, the prohibition of marijuana is far more harmful to environment, economics and society than any other policy being imposed.
Abuse of marijuana or anything else is a symptom of deeper unhappiness/imbalance. Finding out what the root cause of substance abuse is is the only way to recover balance. I sincerely wish you the best of luck in finding out why your child feels the need to use marijuana so much.
I am the father of a young son. If he chooses to use Cannabis when he grows into adolescence, there is nothing I will be able to do other than to make sure he recognizes and accepts full responsibility for his choices and how his life turns out. If he doesn't know the difference between right and wrong by the time he's 14, there won't be anything I can do about it.