One of my favorite riddles

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 the things you do might just as well be turned 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, 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 person.

You may run me for profit or run me for ruin -- if 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?