Now, I understand how hard it is to keep yourself from worrying about those things; I've been there.
First, try distracting yourself. Doing something you usually enjoy could help take your mind off of things, even if only slightly.
Try to think of the times that you were worried about him, and how he eventually came back and proved that your worries were untrue. Try to remind yourself that he could just be busy, which is why he isn't replying, and that he'll get back to you as soon as he can.
And lastly, I think that maybe you should share your worries with him, if you haven't already. Perhaps he can comfort you. It's good to be open and express your worries with the one you love.
In the end, it's important to do your best to keep positive thoughts, though I know how impossible it feels to make the negative thoughts go away. But it's still possible, and I believe you can do it. :) Keep those positive reminders that he's okay in your mind.
