What is the answer to this riddle?

I need help with this riddle. I am your constant companion. I am your greatest helper or 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 autmatically. I am the servant of all great men; and alas, of all failures, as well as Those who are great, I have made great. Those who are failures, I have mde 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 place the world at your feet. Be easy with me and I will destroy you. Who am I?