The fact that you had shown up to the session (you could have attempted suicide beforehand) and had agreed to a hospital admission shows you had some insight and were taking steps to keep yourself safe...but made an impulsive decision and won't take responsibility for it.
If she had let you leave without a plan and without checking in then this would be a different case.
I feel quite sorry for her being put through this.We give our therapists so much power we forget we have our own and they are not responsible for us or our actions outside of the sessions...thats our job.