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, 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 great, I have made great.
I am not a machine, though I work with all the precision
of a machine.
Plus, the intelligence of 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?