Strong:
* Stressed
* Anxious
* Uptight
* Tense
* High-strung
* Rigorous
* Demanding
* Intense
* Fussy
* Picky
Other:
* Difficult
* Uncompromising
* Strict
* Unforgiving
* Prone to anger
* Short-tempered
* Argumentative
The best antonym will depend on the context. For example, "strict" might be a better antonym than "stressed" if you're talking about someone's personality, while "stressed" might be better if you're talking about their state of mind.