Focus
Stray thoughts, the ones that have nothing to do with what's in front of you, quietly wreck your concentration and keep you from finishing anything. Meditation builds up your ability to pay attention and hold it there. You can start with something small: sit for five minutes and watch your breath without trying to slow it, deepen it, or alter it at all. The whole task is to stay calm and refuse to let anything pull you off course.
What you practice while meditating follows you out into the rest of the day. Dull jobs like paperwork and household chores go faster when you bring that same one-thing-at-a-time attention to them. Right now, writing this, I've shelved every other project that keeps tugging at me and I'm giving all of it to this one piece.
I've said this before, but it bears repeating: write down everything that needs doing and keep that list somewhere in plain view. A visible list clears out the mental fog. There's real comfort in knowing nothing important is going to slip, and you stop having to nag yourself over and over just to keep those items from vanishing.
The clutter in your mind is partly made of these intrusive thoughts, a lot of them negative, that run on a loop and never shut up.
Get good at this and you'll notice something odd start to happen. You finish more, and you do it in less time than you used to. That speed is a reward from meditating that most people never see coming.