What was your initial goal? Or maybe you didn't have a conscious one to begin with and had an attraction or something like a curiosity for the subject. Once you covered enough ground you satisfied your curiosity and your interest faded. I think this is a very natural outcome and depending how you approach your learning subject you can achieve different outcomes. Try learning in a class setting where there is a set curriculum and where you could approach your learning in regular and consistent chunks. It may be boring at first but it could get much deeper into the topic.