One part of it is that

"The reason people find it so hard to be happy is that they always see the past better than it was, the present worse than it is, and the future less resolved than it will be.”
.
While this is true for many people another is that happiness being an emotion it comes and goes as it will
It does not arrive to order
It does not heed our commands
Although many behave as if it should
Happiness is though not what we are here for
Certainly we prefer to be happy and not to be unhappy
And at the same time we are aware that unhappiness is often where we learn our lessons and grow.
Happiness is therefore an emotion that seems unpredictable
Given that the times we are now in are more volatile than they have been in recent decades.
And likely to stay that way.
Leading us towards more stress
Happiness is likely to be more difficult to find the more stressed we are.
Unfortunately current times are likely to remain stressful
So it might not be a bad idea to focus on what needs doing to have more rather than less happiness.
And yes it is possible to achieve this even in stressful times.
Start by accepting that happiness and unhappiness are both a natural order of things.
One follows the other as surely as night follows day.
Next decide to let go of the past it is gone, over, finished.
So let it go do not keep thinking about it.
Do not hang on to it, it is gone over done with accept this.
Decide also that having chosen your future direction in terms of what you want to be doing
Where you want to go and then stop thinking about it.
Trust that life will show you what is best for you
Not necessarily what you want, but what is best for you.
Accept this as nature taking you through those lessons you are here to learn
If you can do this letting go of your perceptions and that life should be like this that or the other
Then life becomes more enjoyable
Choose to live in this moment now
Live as if each day is your last
Doing this means no regrets
No more worrying, taking each day is as it is
If you can do this then happiness will come along more frequently
Unhappiness will be less and that quote at the top will no longer be true for you.