Feel so guilty about missing his call

My boyfriend is in boot camp, and I haven’t talked to him on the phone in a couple weeks. I was taking a short nap today and when I woke up, he had tried to call me and left me some messages. He’s not mad at me, but I feel so awful. I already sent him a letter apologizing and everything, but I feel so guilty and feel like I’ll never be able to forgive myself. He said he’ll be able to call me in a week or two, but I still can’t believe I did this. I feel like the worst girlfriend ever. I’ve been crying on and off for an hour, and every time I think about it, I start bawling. I feel so terrible.