How much should I worry about drug abuse?

Wow, tough question! That is a very difficult question to answer as drug abuse can vary on so many different levels with so many different drugs. Abuse of caffeine (yes, it's a drug) is much different than abusing cocaine, heroin or other illegal drug. A good start to figuring out whether you need to worry is if using the drug causes any consequences in your life. If so, then yes, i would say you need to worry.
Don't worry. Just get support and treatment!