Can you guess who I am?

Can you guess who I am?

I am your constant companion.
I’m your greatest helper, or your greatest 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’ll do them quickly and correctly.

I’m easily managed.
You must merely be firm with me.
Show me exactly how you want something done
And after a few lessons, I’ll 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.
And those who are failures, I have made failures.

I’m not a machine, but I work with all 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.
Just take me, train me, be firm with me.
And I’ll lay the work at your feet.
But you be easy with me, and I’ll destroy you.

Who am I?