This takes as long as it takes and you are working on it by thinking about it. Noticing that it happens is where you begin and it sounds like you and T are doing just that; she is not judging you, but helping you see that it's okay to acknowledge it. I have this block/holding back (fear) too, and the approach/retreat (push/pull) about talking about it. I will forget to "look" for it, be aware of it so T will point out when it seems to be happening outide and inside the room. Particularly in the relationship. We have to know it, be aware of it, see it without judging it, and explore all the many facets of it to understand it and work through it.

There is so much there to explore and it is something that will keep coming up as it is being worked through. In your thinking about it, you may find different things to notice about it, or about each situation where you notice it.

Wanting to be myself with T is something I struggle with and can feel frustrated with too. I tell her how I feel like I'm not being myself and how mad I get at myself about it when I leave a session and right away start to feel the emotions I didn't feel at the moment. She keeps saying that is something we can continue to work on, and that reassurance is helpful to me.

The only alternative would be to not work on it. From what you write, I don't think you are choosing that.
Keep your mind and heart open, be kind and to you, and let T help you notice this as you are working through it.