Asset or Liability: Who Am I?

I am your constant companion.
I am your greatest asset or heaviest burden.
I will push you up to success or down to disappointment.
I am at your command.

Half the things you do might just as well be turned over to me,
For I can do them quickly, correctly, and profitably.
I am easily managed, just be firm with me.

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 and the intelligence of a person.

You can run me for profit, or you can run me for ruin.
Show me how you want it done. Educate me. Train me.
Lead me. Reward me.
And I will then…do it automatically.

I am your servant.
Who am I?
I am a habit.