At things that only exist in the mind, like, say, pain.

Except that's not even close to true.

In what sense? Pain is a complex emotion triggered by various simple sensations (very hot, very high pressure etc). But remember that people with certain kinds of brain damage can feel these same sensations, but not pain. To them these just don't register as painful. Other people feel pain in limbs that they no longer have, so not triggered by any sensation at all.

Also, even beyond medical issues, different people perceive pain very differently. Hot peppers are perhaps the clearest example of this, where people accustomed to them feel the same heat, but not the same pain as someone unaccustomed.

In the sense that it's a physical process triggering mental reactions.

Pain is the mental reaction, not the whole process (of course, mental reactions are themselves physical processes, but that's a different discussion). When you take an opioid, the pain goes away, but the physical process that was previously causing it doesn't.

This is such a one sided dogmatic view of how the body works and heals that I honestly don't know where to start. I've seen enough evidence of bodies healing themselves just from changed minds. The truth is the synthesis of western scientific reasoning and eastern mysticism and holistic thinking. The problem was always blind faith, dogmatism, which is exactly what you're doing.