You actually described your problem: "constantly changing subjects". Stop it!

Deep work is not the goal, an obsessive interest in a single subject leads to deep work ethic. When you are truly intrigued and/or passionate about something then deep work follows. Not the other way around.

One clue to finding your niche is to notice what you get very angry over and want to do something about it. Having found that is the beginning of finding the "juice" and the rest follows.