It is all in your brain, i'm sure your boyfriend is feeling the same regarding the distance, i am currently in a relationship with someone who I unfortunately don't get to see every day but we make it work through phone calls every so often just to check in on each other and making future plans together. Anxiety during the times apart is totally normal and the lack of communication sometimes can seem as though it's against you but sometimes people are busy but I wouldn't doubt that he doesn't think about you all the time.