There are probably a number of causes, but coffee may be one of them. Don't be radical, but if you're having too much coffee, I'd suggest cutting down to just one coffee in the morning. It will take a bit of effort but may help you concentrate.
Then I suggest you try to focus on finishing things as the main goal. You have to get rid of the addiction to the rush of novelty, but you can get addicted to the rush of publishing, with obviously positive results. In order to do this, lower the bar enormously. Take only TINY projects: writing a short article or even a tweet. Writing a tiny piece of software that does something. Cleaning up one corner of your disk. It is key that you accept average or even poor quality, that shouldn't be a consideration. And it is very, very helpful that you publish the result: post the essay on Twitter, Facebook, or a blog. Upload the code to github and share it. Obviously, cleaning up your disk drive isn't so amenable to be published, so maybe you want to write a line in an "achieved.txt" file.
Make sure we are talking really small projects here, and that we are not expecting anything from them but completing them. They will be small and mediocre. No problem at all! You can't solve everything at the same time. Make things small enough that their scope fall under your current reach, which you said is tiny - so make tiny thing! Err on the side of caution. You want to make sure you complete them. Half-an-hour projects are perfectly fine here!!
Do this for a week and recap.