The Riddle: "Who am I?"

I am your constant companion. I am
your greatest helper or heaviest burden.
I will push you onward or drag you down
to failure. I am completely at your command.
Half the things I do you might as well
turn over to me and I will be able to do
them quickly and correctly.

I am easily managed - you must merely
be firm with me. Show me exactly how you
want something done and after a few
lessons I will do it automatically. I am the
servant of all great individuals and, alas, of
all failures as well. Those who are great, I
have made great. Those who are failures,
I have made failures.

I am not a machine, though I work
with all the precision of a machine plus
the intelligence of a human. You may run
me for profit or run me for ruin - it
makes no difference to me.

Take me, train me, be firm with me,
and I will place the world at your feet. Be
easy with me and I will destroy you.

Who am I?

I am Habit.