Who Am I?

I am your constant companion.
I am your greatest helper or your heaviest burden.
I will push you onward or drag you down to failure.
I am completely at your command.
Half the things you do , you might just as well turn over to me,
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 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 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 put the world at your feet.
Be easy with me , and i will destroy you.  Who am I?
I am HABIT!