I am not sure how common is this, but I experience loss of interest pretty often. Unlike many people, my problem is not in starting things but in sustaining the interest.
One day, I decide that I am going to do a specific project/task. I will be highly interested and full of energy to get it done. After spending some time in it, I start thinking that the output may not be really worthy enough. I get doubts about the utility of the project. I lose the interest. Other projects appear more luring and I get distracted. I abandon the project and jump into new one.
Not, just any project. Even tv series/websites/friends/relations everything is the same. I lose the interest much before the task is finished.
How do I ensure that I don't lose interest in a thing that I started and stay till the end? I am not abandoning it when I am stumped by a problem. I am stopping it much before just because of my thoughts.