Riddle Me This?

To the one who solves the riddle, you win this week’s prize (the prize is also a mystery)! After someone solves the riddle, I will finish the blog.

No cheating, don’t ask Jeeves or Google. Use your noggin!

Here we go…..

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 at your command.
Half of the tasks that 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;
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, but I work with all the precision
of a machine, plus the intelligence of a person.
Now you may run me for profit or
you may run me for ruin.
It makes no difference to me.
Take me, train me, be firm with me,
and I will lay the world at your feet.
Be easy with me and I will destroy you.

Who am I?