Maybe if a letter feels unnatural in some way, write something more in the way of bullet points or an outline? It may still feel "you" the next day because it is still just an outline of your general concerns and leaves you free to talk with your T about it any way you wish. If you read it to her, it may be even clearer and more helpful than a letter. It doesn't place you in such a vulnerable position initially too and maybe be better for ice-breaking and nerves.

Also, consider becoming more at peace with silent periods in therapy, less anxious about filling space. Take a deep breath. It is your time for you to work towards feeling better. If you are worried doing something wrong by not talking, then you are doing the opposite, feeling less okay. Good luck. I remember feeling in a similar way.