Who am I?

I am unashamedly copying this from Jay Osmond's Blog - and he's copied it from somewhere else - and just tweaked it a little bit. Guess who this is?

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 as well turn over to me,
And I will be able to do them quickly and correctly.
I can be managed, but you must 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 the 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 the precision of a machine,
Plus the intelligence of a human.
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 HABIT

Habit got me into drinking in the first place. I'm trying to learn new habits and so far - day 17 - it's working.