Being attached to a person too much and having them constantly on your mind ?

Hello everyone

I am a person who doesn't likes talking too much to people . But when I get close , it's a different scenario. I am in a relationship right now and I can't get my partner off my mind . Even when I am sleeping, studying, eating or doing anything. The first thought is theirs in the morning and the last of them . And I need constant validation that they belong to me . I know that not everything is permanent, but still I get too afraid. Just by the thought. How to get over it ?