Nah, you ain't being a bad boyfriend. Well, here is what I say:
You love her, so you miss her when she's gone, even though it is for a couple months or so. Nothing wrong with that. And, uh, you miss her a lot, but you are also happy for her, and that's good.
And, you are not too young to be depressed over a girl, I don't think, because I'm the same way (except that I love her but she doesn't love me