Be patient with yourself and take as much time as you need to open up. I like the suggestion of writing things down before session and giving it to your T to help you get started. Trying to face things under the influence of anything doesn't really provide any closure, the true feelings are masked and never really faced.
It took me a very long time to open up in therapy, never speaking above a whisper and trying so hard to disappear. I used my journal and sketchbook to help get things started.