I agree with @InTheAirTonight as you have two contradictory actions, which will ultimately end in disaster. If you are willing to CTB, you cannot afford to divulge it to a mental health professional or anyone in a position of authority who has the power to incarcerate you/take away your liberty. However, if you decide not to CTB (aka changed your mind) then feel free to be honest with your doctor, but keep in mind of the potential consequences if your doctor believes you to be a danger to yourself or others, he/she would (by law and profession be required to) report you or have you committed against your will. 
To my understanding, logically speaking, if one is intending to CTB, the last thing one wants is for someone to intervene, save, or prevent one from being able to carry out his/her CTB attempt.