Can You Solve This Riddle?

I am your constant companion. 

I am your greatest helper or your heaviest burden. 

I will push you onward or drag you downward. 

I am completely at your command. 

Half the things you do, you might just as well turn over to me and I will be able to do them quickly and correctly. 

I am easily managed--you must merely 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 all great people and of all failures. 

Those who are great, I have made great. 

Those who are failure, I have made failures. 

I am not a machine, though I work with all the precision of a machine plus with all the intelligenc 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? 

HOW ABOUT A LITTLE HINT:

 "_____ is a cable; we weave a thread of it every day, and at last we cannot break it." Horace Mann 

I am HABIT.