I lived this, also prior to knowledge of SI.
It was equally as bad for me as DDay, because Perv knew how much I wanted M and it is one thing that pushed me to D. Knowing the extent of cruelty he exhibited, but I will not to go into that.
When he "returned", he had not really officially ended it with OW, but led me to believe so.
The first clue was his behavior. He did not act like a man who was reconciling for more than two days. There were two days that were like a dream for me, ecstasy, that my life would get to continue what I planned and worked at for 20 years.
But his actions, his behavior, his demeanor...weren't what they once were, weren't consistent, weren't always warm towards me. Sometimes, very much so (this was acting) and more and more as the days went on, he became this monster-person, even making fun of me, saying, "trigger, trigger, trigger" in a low sing-song voice.
Previously, he had been a very chatty man who adored attention and hearing himself talk. During those days, he barely grunted in conversation and spent much of his time in solitude (he was texting OW) or darting around if we were out, never, ever in the same room with me.
The distance he kept from me was palpable...I kept thinking, why isn't he getting closer? He'd sit on a different couch or not near me at meals, but be "reconciling?"
It began to happen where he would say things that wouldn't add up. It was the holidays, with gatherings and extra relatives and he would tell me how excited he was to see so-and-so again, but make a last-minute bail out...he was meeting OW instead when DD and I would go and we would cry because we both wanted him with us.
I suspect I could write forever on it and I'm sorry.
My neighbor is a BS too and she says "Oh, you just know. You feel a drop in your gut and have to sit down and can't speak, maybe...you know. They are not themselves and things don't add up.
He would delete his cell phone's log and he would unblock the mobile phone website when I would ask to look. He would make every excuse not to get tested for STD's so we could "get closer" and I know it's because he had no intention. He kept saying "ok" but making ways not to do it, like other things for true reconciling.
You will just...know. And I hope you won't ever.
A person is a person, no matter how small. -Dr. Suess
Perserverance and spirit have done wonders in all ages.