Hi AnxiousGirl, I have had this problem too at times. I have never e-mailed T but what I have done is take a pen and paper an hour or so before my session and write down a few points to bring up. Or if I've done some journaling already during the week, I take it out and have a read over it and try and summarize it. That way when I freeze up or get stuck I just take it out and remind myself. And if I'm very stuck I go back to the longer pieces of writing while T waits patiently. Sometimes I even start the session with the page under my nose. At least then I am still in control of how much I bring up. Whereas with emails, that is taken away once you click send.
Don't worry about the structured conversations you hear between Ts and clients while you wait. Like you said, you are only 20. I am 5 years older than you and only just learning how to handle myself. I would not have had the strength to be in therapy 5 years ago so give yourself some credit. The important thing is you are trying to push through this problem and you will get there.
Oh and I have also got frustrated with having to depend on my journal and I've even refused to take it out at times. But I think knowing that the option is there is re-assuring and it stops me from freezing and thinking the session is doomed