The world has infinite amount of options for us. Instead of reading this you might be doing a million different things. What's the opportunity cost here?
This is what you loose by not doing the other things. By deciding to do one thing you give up - consciously or not - doing the 999 999 things that are possible. Have you thought about it this way?
By reading this you're telling me you're not plowing your garden, not coding a new app or not making music, etc. That's why deciding is called deciding which comes from latin and means "cutting-off". You're cutting off all the other options in favour of the one you decide to do.
The Successful vs. The Rest
Have you noticed that some people seem to get 10 times more things done within the 24 hours than others? Or have a happier life, or are wealthier or have more of whatever you call success.
Well, guess what. It's not that they're working faster. It's not that they put in more hours or that they're talented beyond comprehension. No. What it means is they're focused on what they want.
Focus Is The Antidote
Out of the 1000000 things you could be doing there's very little you can actually do. There's so much you need to ignore that it's mindblowing.
The danger here is that out of the gigantic space of possible things random stuff will get onto your todo list. Just a tiny percentage of those can wipe your real goals out. It's enough to forget where you're going for a second and you're toast.
That's why you need clarity about your priorities. The less the better and having 1 is best. It's OK to change them, but you need to do it consciously and not too often. Between those changes keep an eye on the priorities and work on them.
Update: I just realised I've "written this post" before. Just from a different angle.