Hello. I have found that my special interest changes every 2-3 years. That may not be your case. But what I have found that once I basically know everything of that topic, I get bored, and change topics. My first obsessive interest was tigers, when I was 8-9, a few years after I learned to read. Once I knew everything about tigers, I found something else. My mind works in such a way that I want to learn as much as I can about everything, but mainly focus on my special interest due to ASD.
For you, I am not saying that you should learn everything about that topic, but make yourself busy in other ways. Like, make yourself busy in something that relates to your topic that you are obsessed about. For example, I write Fanfiction. When I was obsessed with Stargate SG-1, I wrote Stargate fanfiction. A few years later, I became obsessed with NCIS, so then instead of writing Stargate Fanfiction, I wrote NCIS fanfiction. I combined two main interest, and one faded eventually. Maybe that will work?