Mental stress is the most general kind of stress.
The stereotypical situation for mental stress is when a person spends years doing a job he or she despises. Two lines of thought clash constantly in the mind of such a person: "I don't want to!" and "I have to!".
Deeper analysis and brave decisiveness are required to overcome a situation causing such stress. If you have to make a choice between A and B, and you dislike both choices - open your mind and find the strength to create a third choice.
It is unfortunate, but many people deal with mental stress by simply killing of a part of themselves and creating a delusion that they indeed want something that they don't. Such resignation towards the choices in life can cause life-long damage.
Most kinds of mental stress can be solved by proper prioritization.