There is only one thing I have held back from therapists in the past, and even that would have been far more beneficial to have said than to have kept to myself. That one thing was “I am actively planning my suicide.”
I held this back because I know that the only two pieces of information a therapist is legally allowed to report (other than child abuse) are 1) if you are planning to hurt yourself or 2) if you’re planning to hurt someone else. If they find this out, they can work to have you held in a hospital for 72 hours to assess your mental health and try to make you safe again.
I held this back because I didn’t want to be stopped in my progress toward suicide. I had already decided and was putting the necessary steps in place for a successful exit. Long story short, I lived.
Now, I tell my therapist everything, because I need her feedback on sensitive subjects, like my failings with my son, my frustrations with my husband, my sometimes injudicious choices, and other personal weaknesses. She is an excellent sounding board and it helps me tremendously to share things with her that are hard just to say out loud. It destigmatizes these topics that I am ashamed of and helps me cope more healthily and realistically with them.
I hope you have a therapist you can trust, and I hope you will choose to trust her/him with your darkest secrets. There is nothing you could say to a therapist that would not be in your own best interest.