What's wrong with me?

I'm currently at school in another country from my S.O. I trust him but I don't understand why I'm still continuing to question what he is doing away from me and what he might be doing when he isn't responding to texts etc .  I feel like I'm obsessing over him and it's really putting a strain on me. Has anyone experienced this? Is there anything that I can do to stop thinking about him all the time.