Who am I?

Ive run this before, but thought it was worthy of posting again. Here ya go!

"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.

I am not a machine, though I work with all the precision
of a machine.

Plus, the intelligence of 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 HABIT."