That's me, too. I obsess sooooooo much with something, learn everything FAST (I learned how to speak almost fluent Japanese in 1.5 months lol), and then one day, just wake up and I'm not obsessed anymore. However with me, there's ALWAYS an obsession lol. Like, I don't just stop with one and then I'm not obsessed anymore...something always replaces the other obsession.
Like, I've been obsessed with something for 8 years now, and when people make fun of me for it (especially because I'm an adult now), and when they ask why I just don't stop, they don't seem to understand that it's kinda not under my control and I can't just stop it, and it tends to really upset me when people can't see that I can't just stop it and forget about it just like that, because they make fun of me for liking the things I'm obsessed with a lot. But then when I start doing stuff involved with what I'm obsessed with after being upset, I feel so much better lol 