There are so many opportunities in our lives, and so many demands. This is not a complaint: it's a blessing. But it also means that focus is difficult.
We are called upon by family and friends.
We are faced with the mundane requirements of life: paying bills, buying food, cleaning house.
We may have multiple jobs.
We may have hobbies.
And, of course, the entertainment options of the present moment are countless: TV, movies, books, the internet, music, art, sports, travel.
If you don't have good momentum on a project, it can easily get lost in the other demands for attention and for focus.
I don't know that I have an answer here. I know that I am easily enough distracted from my own projects. When the only responsibility is to myself, it is more easy to be distracted.
Oddly enough, something I forget about writing is how enjoyable and rewarding it can be. It's easy to think of writing as work, and then to associate with it the negative connotations that the word "work" carries in the culture of the US. In particular that work, while the "right" thing to do, is not supposed to pleasurable.
But that's not true of writing. Writing is hard work; it requires effort; it can be immensely frustrating. But it can also be rewarding--not just because someone else reads and appreciates what you've written--but because of the very pleasure of the act and the learning and growth related to working through a problem and finding a positive resolution to it.
Learning is pleasurable; mastering a skill is pleasurable. It is a real, experiential pleasure. Writing a sentence that has particular beauty, or crafting a paragraph that resolves an argument in an elegant or clever fashion-these are actions that are pleasurable in the same way that playing a musical instrument is pleasurable, or skillfully mastering an athletic endeavor--though I'm not a golfer, I think it is, perhaps, an apt parallel: the sport's allure is not so much in the gross expression of physical power--as for a runner, for example--as it is in the fine skill and thought. Billiards might be another parallel. Or chess. To a lesser extent any game of intellect; I say "to a lesser extent" because writing, given its difficulty, seems to provide the greatest opportunity to overcome difficulty: that which is too easy to overcome is not rewarding to resolve.
When faced with many distractions--ones of lesser pleasure, more easily achieved--if we can only remember the potential for pleasure is so much greater if we can engage in the work and overcome our difficulties, we will more likely be able to put the distractions behind us.