Who Am I?

Who am I? I am your constant companion. I am your greatest helper or your heaviest burden. I will push you onward or drag you down to failure. 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 men. 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 all the precision of a machine. Plus, the intelligence of a man. 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 put the world at your feet. Be easy with me, and I will destroy you. Who am I? I am a HABIT!