Can you identify what I am?

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 just 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 men; and alas, of all failures, as well.
Those who are failures, I have made failures.
Those who are great, I have made great.
I am not a machine, though I work with all the precision of a machine, plus the intelligence of a man.
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? Iam a habit.

– Dr. Dennis P. Kimbro