You do not have to tell them everything on your first go. You are in control, its your time, your session. It's probably wise to be cautious, until you've had a few appointments & you are confident that you trust this person and you feel safe.

You don't have to go in saying I think I have PTSD. People go to therapists for all sorts of reasons from my mild unhappiness or goal setting or more severe illnesses.

You may want to approach by saying I'm unhappy with my life, and there are some things in my past that I want to discuss but not at my first session, I'm not ready yet. A good therapist will totally respect that, and go at your pace.