It may be very difficult for some people to stop using substances because using serves a certain purpose. For example:
If a person uses a substance to cope with physical pain, they may be fearful of their pain returning and the effect this will have on their day-to-day functioning if they choose to stop using.
If a person uses a substance to help them be part of a social network, they may be scared they will lose their social circle if they stop using.
If substance use has become a way of life for a person, they may wonder what life will be like without the substance. They may be scared and anxious of what they will lose by stopping their use.
Some people may also be worried about the withdrawal symptoms they will experience if they suddenly stop using.