You are your worst enemy. In many instances, you're aware of it; in other instances, you are not.
You are your worst enemy when you react without thinking. When you fail to ask yourself questions, like: "What's the best approach here?" or "Why am I angry over this?"
You are your worst enemy when you are constantly frustrated with the people around you - your coworkers, your significant other, your family - without ever asking yourself, "What can I do to make the situation better?"
You are your worst enemy when you get depressed because you lack the consistency to maintain your diet, your exercise routine, your blogging, etc.
You are your worst enemy when you get overly excited about achieving your goals, when you know deep inside that you'll probably stray from them.
It's not until you subdue yourself - until you realize these weak points and either overcome them or make them irrelevant - that you'll actually start to make progress or create peace.
This is probably the hardest task to do, but it's the only real one that matters.