riddle....can you guess it? (i did not)

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?

**HINT**

i guessed one's body...i was wrong.

alec said a trainer...he was wrong.

my mom guessed one's concience...she was wrong.

what's your guess??