Habit

You may know me I'm your constant companion. I'm your greatest helper. I'm your heaviest burden. I'll push you forward and upward or drag you down to failure. I'm at your command.
Half the task you do might just 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'm the servant of all great individuals and of course the servant of all the failures as well. I've made all the great individuals who have ever been great and I've made all the failures too. But I work with the precision of a marvelous computer, with the intellect of a human being. You may run me for profit, you may run me to ruin, makes no difference to me.
Take me, be easy with me and I'll destroy you, be firm with me and I'll put the world at your feet.
Who am I?
I'm HABIT!