The Renaissance philosopher Michel de Montaigne (1533–1592) spent most of his days in his library on the third floor of a cylindrical tower at one corner of his castle, 30 km east of Bordeaux.

He had about a thousand books on philosophy, poetry, history and religion, which were arranged on semicircular shelves against the wall of the tower room.

He also had 57 short inscriptions (taken from the Bible and the Classics) painted across the wooden beams which supported the library ceiling. One of them (by Sophocles) was :

When I read this in Alain de Botton's book The Consolations of Philosophy my mind immediately cast back to my Dad's pocket diary (the popular Dutch "Success Agenda") of 1981 (the year he died). I still have it, it includes, besides addresses, birthdays of his family, friends and staff, etc., two pages of short sayings my Dad had collected from various sources over the years. One of them read :

Why this negative view of the human mind (so contrary to most philosophers) ? I understand and appreciate the intention above statements attempt to make, but taken literally they are of course wrong, because they merely shoot the messenger.
It is not the mind which causes us unhappiness or any other emotion. It are our circumstances (using the word in its widest possible sense) which are the cause for our emotions. The mind merely interprets the circumstance and passes it on to trigger the appropriate emotion.

Without a mind (assuming that the rest of our brain still functions) we would feel no emotions at all ! We would merely live like zombies.

I am sure you have experienced proof of the above "chain reaction", but let me give you a typical example.

About 20 odd years ago, teaching one of my Saturday afternoon bridge classes at my home in Glenelg (a coastal suburb of Adelaide, South Australia) there was a middle aged man in my class who was just divorced from his wife.
"Michael", he said at the end of one afternoon to me "these are the only three hours of the week that I am not in total emotional agony over my present circumstance. My mind is so busy figuring out this wonderful game of bridge that it has no time to focus on anything else!"

Prince Andrei Bolkónsky, in Leo Tolstoy's epic masterpiece War and Peace, is in emotional agony after his broken off engagement with young, beautiful, lively Natasha.
Happiness he explains to his friend Pierre Bezúkhov, is the absence of agony.

Marie-Henri Beyle, better known as Stendal (1783-1842), in his first published novel Armande (1827), expresses the same sentiment through Octave, the main character of the story.

The bottom line about changing the state of your emotions when you are unhappy, distressed or discontent is simple : examine you current circumstance and try to do something about that. Three common scenarios come to mind.

The first one is when there is a circumstance you can do absolutely nothing about, a death in the family, a divorce. In such case try to find temporary relief (like my bridge student in above example) by diverting the mind to something totally different.

The second possibility is that your mind misinterpreted the circumstance. For example you find your girlfriend in (what you believe to be) a far too intimate conversation with another guy. Naturally this makes you unhappy, not to say upset. But as you join the couple you suddenly find out that the supposed contender is your girlfriend's brother. Immediately your emotions do a dramatic about turn.

The third scenario is where you need to change your circumstance. In its simplest form this may involve perhaps talking it out with someone who has upset you with something. This almost always works.
In the ultimate case, where the circumstance is your current life as a whole, there is nothing but either bear it or change your life. A "sea change".
This may be relatively easy to achieve, or it may be agony, like in my own case, where, after 15 years of regularly recurring deep depressions in my former life as a professional geologist, I totally upended my life and started from scratch an entirely new life in music. It was very painful, especially for those so dear to me, and it was not easy, but it cured my depressions and unhappiness forever!