I sketch and write and code regularly. This is, it seems, what I enjoy doing; making things. And I believe I've been this way my whole life. I struggle, though, when whatever I'm making starts getting "serious."
We tend to over estimate what we can achieve in a given period of time. The execution of a seemingly simple idea—apparently, fast and easy when it first comes to mind—ends up being harder (and more time consuming) than we thought.
Let me give you an example. No matter where I am, I like to jot down ideas (or even short drafts) I might someday turn into a short essay. I enjoy writing the draft, and editing it over and over. It is not until a text gets closer to its final shape that I start to freeze.
To finalize anything you make, you usually need to stop obsessing over certain details and decisions you've been postponing, which requires one last push to reach a state you can consider done. Dozens of ideas of things I could be doing come to mind in this part of the process. I tend to stop and postpone the finishing touches for later. If only for a couple minutes, I dive into something else, loose my concentration, and find out that time is up. I need to move onto something else and there's no more time to work on that piece today—let's try again tomorrow.
The resistance hits hard, and you let go right when you just needed one more push. To avoid this, I'm trying hard to mono-task; to do less—better—and to put my sole attention on one problem at a time. From my own experience, taking a break helps, but only as long as you disconnect from mentally taxing activities to come back refreshed for that last push. Go for a walk. Pause. Then, come back and get it done.