alex, how about writing about the topic that seems most pressing or urgent? Same as you would pick your topic for the face to face session. (At least that's how I pick mine--the most important thing I need to talk about at that moment.) Maybe the writing format will even allow you to choose topics you would normally be too shy to bring up in session. If you are stuck, maybe try completing this sentence to get you started: "If I could only say one thing to my T, I would say......"

Since I tend to be verbose, I would set myself a limit on length, probably 3 paragraphs or so. But that's just me.

Good luck.