I also lurk on relationship boards and the scenario you outlined plays out over and over again. Exactly.

On a personal note, I have had many partners accuse me of cheating when I wanted out, and there never was anyone else. I just wanted out. So there can eventually be someone else. 😉

There are two ways to end a relationship. Secure a backup plan before leaving or leave without a plan but hope for something better. You deserve the latter.