For an instant, you feel alone.
At first, especially if it's the first time it's happened to you, you don't really know what it means. Your first reaction might be to ignore it, or obscure it. It's such a tiny little thing, this flaw of doubt. If you pay attention to all the good things you have, all the memories, all the situations you can still make, perhaps the flaw will seal itself, or you will forget where it is and it won't matter.
That doesn't happen. So after a while, you convince yourself that the feeling is just something you need to adapt to. In fact, perhaps it is necessary. Perhaps that vague sense of unease is just a part of who you are. Perhaps all relationships get to this point, in short order, and the trick you need to learn is how to keep the gears turning even on nights when the doubt is absolutely crippling and you are beating at the inside of your own mind, screaming the words YOUR LIFE IS PASSING.
Eventually, it does.