Who am I?

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 - you must be firm with me. 

Show me exactly how you want something done

and 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, and I will place the world at your feet. 

Be easy with me and I will destroy you. 

Who am I? 

I am Habit.