Only Him

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 of the things you do you might as well turn over to me and I will do them - quickly and correctly.
I am easily managed - you must 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 great people,
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 the precision of a machine
plus the intelligence of a person.
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.