I've gone through this. The only difference is that I could point to what was distracting me and taking my motivation: gaming. Playing highly competitive games

I've gone through this. The only difference is that I could point to what was distracting me and taking my motivation: gaming. Playing highly competitive games was an addiction for me (though I denied it for a long time). I wanted to improve my skill, knowledge, and win rate in the games I played, which consumed my attention, and drew me away from work. As you've said, there isn't an identifiable internal or external stimulus which is causing your distraction, so I'll have to be general and subjective with what I am about to say. Perhaps take a look at the actions you take when you are distracted. Do you flock to a specific place or action when you lose interest? This can be the internet, eating, running, driving, etc. If you find a pattern in your distraction, analyzing the issue becomes easier. There's always the possibility that you're doing something you don't like and haven't realized it yet; perhaps a career change or dev change is in order. Changing my environment helps significantly when I want to maintain focus. Getting out of the house increases my productivity and inspires new ideas. So reflect on yourself, your work, you happiness, and your interests and make sure everything is in check. If it is, and you're still losing focus then an Attention Deficit Disorder test could be an option (however many people are over diagnosed for this disorder) I hope some of this helped.