You can't be. However, we can learn to recognize extreme over-reactions, e.g. being terrified while flying or obsessing for hours about an insoluble problem. But a little worry about crashing while flying is realistic and some thought is necessary to know that you can't do much about a problem. They dismiss or deny it. Both need to learn to react differently.
Anxiety is the most common symptom seen in a psychologist's or psychiatrist's office.
In a more limited usage, stress is the upsetting situation and strain is the mental and physical reactions. However, most of us use the term stress loosely for both the threatening situation and the anxious reaction. Thus, the experienced jogger meets the demands of running five miles and thoroughly enjoys it. A person given a promotion is delighted even though it means more responsibility and work. Doing well in school involves the stress of learning what you need to know to get high grades on tests.