Something that is stressful causes stress in others. Something that is stressed feels stress.
One way to remember it: Things are usually stressful. People are usually stressed.
Examples:
The meeting was stressful.
This job is stressful.
Sitting in traffic is stressful.
But…
Everyone in the meeting was stressed.
I feel stressed.
Even my baby got stressed.
Note: “Stressed” also has a different meaning, “to emphasize” or “to give importance to.” For example, “My boss stressed that I needed to follow his advice.” Same word, completely different meaning.