It's a practical impossibility to sell a house against the owners will. Now your reasons for canceling will determine how cooperative your agent will be. If you have been approached by another agent who has promised you something, I wouldn't recommend you cancel. First, that is unethical and shouldn't happen. Next, if they will treat our profession that way, how will they treat you.
If your circumstances have changed and you intend to stay, discuss it with your agent and see what they say.
As an agent, we incur our expenses up front when listing a property. Some of these are hard financial costs, others are significant investments of our time. Please consider this in your decision to cancel.
If your agent has acted inappropriately or unethically, you should be clear to cancel, but may want to discuss it with the agents broker.