The physical and emotional sensation of shutting down is an indicator that you are experiencing a feeling you’d rather avoid. What is that feeling?
Figure it out (hard). And let it be (even harder).
Regardless of how it manifests, a shutdown only means one thing–there’s an emotion you’re not feeling. Why aren’t you feeling it? Because you are afraid. You’re afraid that feeling will be worse than not feeling it. But in reality that’s seldom the case. While you can certainly choose to avoid situations that may elicit a particular feeling, you can’t protect yourself from a feeling once it arises. Any attempt at such protection, even when well intentioned, will only serve to keep you shut down.