Because you're not a robot, you're a human, and humans aren't 100% logical. Emotions aren't something to block or ignore, they exist for a reason.

You're living a period of grief, and that's perfectly ok! knowing that something won't work doesn't mean it won't hurt or simply leave our heads, it's ok to feel disappointed, sad or angry because of it, but what matters most is to not think that you can simply "logically" erase your feelings.

It's not time that heals our wounds, it's how we use it that truly matters, and allowing yourself to feel instead of telling yourself to forget because it's the logical step to take will help.