You seem to be focused completely on the possibility that she'll get another partner and that you'll feel left out or abandoned.
My father is a total pessimist. My Mom would suggest we do something, and Dad would come up with all the things that could go wrong. If we went to the zoo, it might be too hot out, there might be crowds, we might not find a parking spot and have to walk a long ways, it might rain, we might run into traffic, and she'd point out that there was a new baby gorilla, or that it was the day for free rides on the merry-go-round. So, we'd go, he'd grumble, and at the end of the day, he'd have at least somewhat enjoyed himself, and very few of his dire predictions came true- or if they did, it was still better than just staying home and doing nothing.
So, try to be an optimist. What if you met someone wonderful and hit it off, and she got along well with your partner, and they planned together to get you something really awesome for your birthday? What if your partner found someone else, and you got to be good friends with him?