Being in therapy is about forming a relationship based on trust and honesty. The relationship is a safe space for you to practice being vulnerable, honest and open.
If you know there is something big you want to share, but you don’t feel safe in the first meeting, that is okay and is understandable and healthy. You could let your therapist know that there is something important to you that you would like to share, but that you are not ready yet, and you will let them know when you do feel ready.
It is also normal to feel vulnerable in therapy. We sit there and tell the therapist everything. A great thing to remember as you share things that make you cringe is that most therapists have had considerable struggles in their life.