Whatever you send, I suggest sleeping on it and thinking about it before sending it!
I bet your therapist would love to hear what is going well in your life, what you've been doing, what makes you happy. You may be tempted to write about what's not going well -- of course you can, if you want, but I wouldn't recommend it because your former T can't fix it, and she may feel guilty reading it or you may feel sad if she doesn't write back.
Were there any things that the two of you talked a lot about? For example what ElectricManatee said about coming out... maybe getting a job? Going to school? Doing something you've been scared of? I don't know. But therapists don't forget those kinds of things and I'm sure she would be happy to hear an update.