When you are in school and suffering, you can still motivate
yourself to study hard by telling yourself that the 2 years jc/3
years poly/3 years university is going to end soon and you will be
graduating soon.
When you are suffering in ns you also can motivate yourself by
telling yourself that 2 years is ending soon.
But when you are working and suffering, you have lously
collegues and boss, lots of work to do, you know that you have to
work and suffer there till you retire or die.
How do you motivate yourself to work hard?
If you decided to resign to look for a new job, what if the next
job is the same situation- lousy boss and collegues, long working
hours with lots of work to do etc, than how are you going to
motivate yourself to work for long in the company?
You can start by taking things more positively!!!!
Since you are aware that you will always lean towards the bad or
negative side of any situations, try reversing your depressed
situation by finding all sort of crazy excuses to make that
situation into a funny situation. And if you can make it funny
enough to make yourself laugh out loud, then you have already taken
a huge leap into happiness and becoming a positive person.
Don't blame anyone for your own situation, the
problem ALWAYS lies within YOU. 
You see what you want to believe.