Adolescence is a difficult state which affects all family members. I t is compounded in step-families because of the other dynamics involved. Do discipline your teenage child. Don't give up on them. Do go with the flow.
Flexibility is a great asset especially in the sensitive step-relationship.
Do learn to search for something good when plans have to be changed at the last minute. If you bend you will not break. Do have a sense of humor. When things have really gotten out of hand, think of how you will laugh at it in about ten years. Try imagining yourself in the other family members' position.