Suspicion is useful until it starts treating possibility as proof.

Healthy suspicion asks questions. It notices that something could be the case and then goes looking. The failure mode is when the possibility itself becomes sufficient, so that being able to imagine a motive is treated as having established one. At that point suspicion stops investigating and starts convicting, and it will never run out of material.

When you suspect something, write down what would confirm it and what would rule it out. Then look for both. If you cannot name anything that would satisfy you that you were wrong, you are no longer using suspicion as a tool.