Can you guess who I am?

I want to see if anyone can guess who this is. I will reveal the answer in the next post which will be tomorrow. Hope you have fun with this! Maybe you will be better at guessing it than I was....

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 of the things you do you might as well turn over to me

And I will do them-quickly and correctly.
I am easily managed but you must be firm with me.

Show me exactly how you want something doneand after a few lessons,

I will do it automatically.

I am the servant of 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 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

andI will place the world at your feet.
Be easy with me and I will destroy you.

Who am I?