I liked Dynasty333's conclusion, but it wasn't too clear for me.
The main point for the flash would be that you can't rely on someone forever. And vice versa. That was why when you try to part from her, it was the most logical ending. It was what the protagonist wanted anyways. Leigh also needed to socialize with others instead of just with the protagonist. The "glitch" was perhaps the protagonist going mad after deciding to stay with her. So the glitch, the eeriest part of the game, was nothing but a huge huge huge sign that you need to get away from her.
This makes me question what life could've been if I had the same gf for life. That'd be quite a bore now that I think about it.