Who Am I?

I am your 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 you do you might just as well turn over to me and I will 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 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 all 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?