Hey, I too experience this problem quite often. I worry a lot about the other people I care about in my life. From the moment they leave, the worry clock starts ticking until they come home. I constantly fear that they won't come back. I totally hate why I do this. And it's terrible when I try to call them, to put my mind at ease, and when they don't answer--ugh, anxiety overdrive. I've been to the point of wanting to cry and throw up. It's terrible and I'm sorry you are going through it.
Everyone kinda laughs at me, 'cause I do worry a lot--especially about other ppl I love dearly, but they also understand and are very positive and try to be light-hearted about it. Naturally I'm always trying to calm myself, cause in my head it's always a worst case scenario with me.

I read this a few months ago and it kinda helps me a little bit: "When two souls fall in love, there is nothing else but the yearning to be close to the other. The presence that is felt through a hand held, a voice heard, or a smile seen. Souls do not have calenders or clocks, nor do they understand the notion of time or distance. They only know it feels right to be with one another. This is the reason why you miss someone so much when they are not there-- even if they are only in the very next room. Your soul only feels their absence-- it doesn't realize the separation is temporary." You see? We just have overly sensitive souls! I hope this helps you feel better in some small way..