Why do i think about this girl all the time 24/7, is this normal?

It's totally normal to feel this way. If you got rejected theres almost no way to forget right away. Maybe to try and forget by unfriending her on Facebook(ok thats kinda harsh so nevermind) but if she's on ignore her I guess. Poems can help but sometimes they make you feel worse. I would start by not doing things that rimind you of her. That all I have but please dont worry it'll get better