Many years ago in New York a client who was brand new to therapy excitedly asked me, "Am I going to have a breakthrough?" The term "breakthrough" has been tossed around a lot when it comes to therapy, so much so that many people new to therapy anticipate having a life-changing catharsis as a central part of the therapeutic process, an idea that's both exciting and frightening. Powerful experiences in therapy certainly do happen, but therapy doesn't require catharsis. In fact, therapists as a rule try to help people pace emotional turbulence so as not to overwhelm defenses they may rely on. Breakthroughs are typically smaller, more cumulative experiences that over time add up to lasting change. Sometimes they're conscious. Often they aren't.
Here's an article I encountered that goes into the idea of therapeutic breakthroughs a bit more deeply.