How do I design a therapy intervention that actually works?

You can run an exercise from a plan. Designing the plan yourself is a different order of skill, because it asks you to reason from a specific person to the right approach, not just carry out steps. Feeling that this is beyond you right now is honest, and it is exactly the stretch that turns a technician into a clinician. Good intervention design starts from a clear picture of the gap. What can this person not do that they need to, and what is actually in the way, the strength, the skill, the fear, the habit? Once you name the real obstacle, the intervention almost suggests itself, because it is simply the bridge from where they are to where they need to be. The exercises are tools. The design is choosing the right tool for this particular gap. Take one patient and write, in plain words, what they cannot do, why, and the smallest next step toward doing it. That is a designed intervention in its simplest form. Do that repeatedly and your judgment sharpens. You are not missing a talent, you are building a habit of reasoning that grows every time you use it.