One of the things that I tell my prospective clients before they ever sign on with me is that I can only help them to the degree that they are willing to be honest with me about all that they are dealing with because I am not a mind reader and do not want to be one.
The point is that one cannot expect anyone to be able to help another unless the truth is put out there to be worked through.
Trust is the most important aspect of employing a therapist. One needs to trust that the information given will not be used against the patient such as forcing the patient to be hospitalized for observation if this is not required as happened to a client of mine when asked if she ever had suicidal thoughts. She answered that she did and was rushed off to the hospital in a police car for evaluation. She was not suicidal at the time she was asked this question. The question was if she EVER felt suicidal. So her answer was taken out of the context of the question. Understand that this particular therapist did not know this patient this being the first appointment and the action taken within the first 5 minutes of it.
The best bet is to build a relationship with the therapist so you know that you understand one another. Trust is built over time so allow that process to occur so long as you feel that your therapist is hearing what you are saying in the manner in which you mean it. The therapist is respecting your feelings while allowing you to be an invested part of the process instead of infantilizing you because you may have a mental illness (diagnosis given by a mental health provider with the ability to diagnose you). These are important aspects of being able to do the deep work that is necessary to have therapy be a productive endeavor.
So, yes, you need to be honest with your therapist, and your therapist needs to earn that trust from you during the process of engaging in therapy.