The Key to Success

I have a little riddle for you.  The answer is powerful – more powerful than you might think.  It can help you to make and sustain positive change, little by little, to true success in the goals you set for your business, and personal life:

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 the things you do you might just as well turn over to me, and I will be able to do them quickly, 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 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 human being.  You may run me for a profit or turn 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?