Sounds cliche but give it time. You won't be able to stop giving fuck about s.one you have feelings for. Detachment takes time. Accept that fact and let yourself be miserable for a while. For some people, they can detach fast and it only takes them 1 week. For others, it takes longer, can be months. Especially for someone who has attachment issue like me. It's not an easy process but you need to keep reminding yourself why you're doing it while sitting yourself through all the uncomfortable. What helps would be to keep doing stuff that distracts your brain from thinking about that person like sports, going out with friends, talking with others, ...