Zen is a state of "now."
Where Buddhism and psychology overlap (and many post-graduate psychologists - through Jung - are immersed into Buddhist philosophy) is simplified here:
If you dwell in the past, anticipate depression.
If you dwell in the future, anticipate anxiety.
If you dwell in the present moment, anticipate bliss.
On a more intermediate level: "Ignorance may be bliss, but thought suppression promotes superficial cognitive processing."
Zazan is the nemesis of thought suppression. Through Zazen, you simply learn to deal with thoughts as they bubble up from the subconscious mind and move on - or you don't.
Yet another book (please read this first)