Relapse is very common, especially on a day that has meaning. I am guessing you never got any degree of closure in regards to your abuser, if closure is even possible. I don't think there is any greater trigger than contact with the past.

Forgive yourself for what happened Thurs. Don't try to hide it from your T. That's why she's there. If you are afraid she might terminate your sessions, perhaps there is a trust issue. On the other hand, you might be over reacting to the guilt you feel about cutting, and the fear you have of disappointing your T. If she is true to her calling, she will help you through this. If not, you may be better off somewhere else.
Good luck, best wishes. David