The three-way feedback loop
Thoughts, feelings, and behaviors form a closed loop where each element influences the other two. A pessimistic thought (I will fail) triggers anxiety, which leads to avoidance behavior. The avoidance then produces relief, reinforcing the belief and deepening the negative thought pattern.
This interconnection means change at any vertex shifts the whole system. Modifying behavior can shift feelings and thoughts; challenging negative thinking can reduce emotional intensity and enable different actions.
Intervention points across the triangle
CBT exploits the plasticity of this system by targeting whichever vertex is most accessible. Behavioral activation (increasing positive actions) influences mood and thought patterns. Cognitive restructuring (examining evidence for beliefs) reduces both emotional distress and avoidance. Exposure to feared situations (behavioral change) updates both feelings and the thoughts that sustain the fear.
Because the vertices are interdependent, small changes at one point propagate through the system. A person who challenges 'I can't do this' and attempts a task gets evidence to the contrary, which both improves mood and makes future attempts more likely, creating an upward spiral that reverses the original downward one.