Wow what I wish I would have know:

The therapist is paid to listen to you. They are not your friend. They will listen to you until your hour is up.
Sometimes you are going to leave worse than when you came in.
Therapy is one of the most painful yet bravest things you can ever do for yourself.
You will never look at your life the same way again.
Being attached is painful but also worth it at the same time. (My first thought was to say is don't get attached but then I realize I am glad I had the relationship I did with my therapists. I would get attached all over again even so doing so caused me pain.) that is why I edited this from don't get attached.
Feeling alone takes on a whole new meaning especially when you leave the appointment barely hanging on.

I am sure I could think of some more but not at this moment.