First off, I totally understand how hurt this would have made you feel. When you're feeling terrible about everything and yourself and the one person who is supposed to be there forgets about you (and that's painful in itself, to be forgotten,) you're going to have all the negative feelings you ever had about yourself and other people reignited.

Having said that, I think your therapist probably knows how much she screwed up, and I think you might find it surprisingly reassuring to work through this with her. All your feelings are totally valid, but you might be surprised at how they can change when they are responded to therapeutically.

However, if she makes a habit of this (ie, ever does it again), or tries to minimize your feelings rather than helping you through them, then changing therapists is advisable, IMO.

One of the big things in therapy is consistency and reliability. Forgetting people undermines all of that.