Be assured that you will not lose control, or say or do anything you do not want to whilst hypnotised and you will be aware of your surroundings.  You could terminate the session at any time. In fact to make changes you need to want to make those changes. 

Hypnosis is a state we all go in to regularly, for instance when we are engrossed in something, it is also the stage we go into just before falling asleep.  The purpose of hypnosis is to access the subconscious mind, the part of our mind which ensures we keep our habits, whether they are of benefit to us or not, and to change our responses.  Often suggestions for change are sufficient to make the desired change, if this does not help then it may be necessary to go back to the event responsible for the behaviour.