I'm not sure I quite understood the ending, it got pretty abstract.
Either option, him getting into his own head or his suspicions being true, are equally possible so in retrospect the open ending fits because it reflects the protag's state of mind: none of the information presented to him gives him a clear answer and we are made to suffer with him. I will say one thing though, if you've been together for years that's gotta mean something, he's coping now by convincing himself there never really was; paranoia is marring his view of his past, and his present for that matter. Depression, too.
Hats off, I'm always impressed by short stories that manage to pack such a punch so neatly, and the fact you built it all in only one month... Good stuff