Take a breath!! It's ok...really.
As long as you don't say anything threatening about yourself or others, they won't commit you. Would it be so bad if someone got in touch to check in with you? Maybe she would be allowed to call. It could be a big relief.
You could always bounce letter ideas to PC people and they will honestly tell you if it sounds threatening or not.
Chances are, a letter will sit in a mail slot until she returns, but she will know on her return that you need help.