In his book The Power of Now, he writes that we require clock time thinking to actually exist and achieve our goals in life. With clock time, we remain mostly in the present, with our goals and plans presently in our minds while we make logical choices about how to fit all that into the time available to us.
However, psychological time draws our mind away from the only period in time that actually exists: right now. We immerse ourselves into narratives about past guilts or wrongs, and get carried away with the feelings that arise. Or we fantasize about personal narratives of victory, power and wealth. Or wallow in anxious worry about all the horrors that could befall us in the future.
'Only live in the now' means primarily to give up meandering about in psychological time.
Regarding your other points - the past and future technically do not exist. Only the now exists - living only in the past or future would require one to be completely content in living in their own fantasy world.