What actually changes over months of therapy?

Rarely a single insight. What people more often describe is a gradual widening of the gap between a trigger and a reaction, so there is space to choose rather than only react. Patterns become recognisable while they are happening rather than in hindsight. Self-talk shifts, often slowly enough that you only notice when something that would have derailed a week no longer derails a day. Progress is usually uneven, with plateaus and setbacks that are part of the process rather than evidence it is not working. Reviewing goals periodically with your therapist helps make the change visible.