Your Best Friend or Worst Enemy

You may know me; I’m your constant companion.

I’m your greatest helper; I’m your heaviest burden.

I will push you onward or drag you down to failure.

I am at your command.

Half the tasks you do might as well be turned over to me.

I’m able to do them quickly and I’m able to do them the same every time, 

if that’s what you want.

I’m easily managed; all you’ve got to do is be firm with me.

Show me exactly how you want it done, and after a few lessons, I’ll do it automatically.

I am a servant of the great men and women, and of course, servant to the failures as well.

I’ve made all the great people who have ever been great

And I’ve made the failures too.

But I work with all the precision of a marvelous computer,

with the intelligence of a human being.

You may run me for profit, you may run me for ruin; it makes no difference to me.

Take me, be firm with me, and I’ll put the world at your feet.

Be easy with me and I will destroy you.

Who am I?