Do you think I could tell my gf about my new obsession?
Why do you want to tell her about your new obsesion?
Because I worry that it will be destroying our relationship if I don't tell her.
I can see your point, but it is certainly concerning if you are very obsessed with something. You should perhaps try toning it down a little bit, Otherwise the obsession might take over your whole life and your relationship too.
Is it fine if I do it as long as I don't let it take over?
Well yes, because then you would not be obsessed over it any longer. Either way, let your wife know so you can both get it under control. You can depend on your wife, she'll help you get through this.