WHO AM I ?

Ok..so I decided to do something different...
I'm posting a riddle to see if anyone can actually answer it.. In all honesty..if you get it right--we are either getting married, or your my new best friend ! :) COMMENT BACK WITH YOUR GUESSES! Good Luck! [think hard]
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 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 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 human.
You may run me for a 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?