I was in a similar situation to yours. The man I was seeing was separated from his wife when we met. He told me I was the only woman he’d dated since then. We were together for a year.
As time went on I had an inkling that he was up to no good but when I questioned him he denied everything. He tells lies as easily as most people breathe and has gaslighting down to a fine art. Unfortunately for him a series of weird coincidences led me to discover that he had a long term partner who he’d been with for at least five years: he’d started seeing her when he was still living with his wife.
A couple of weeks before Christmas I told him I knew what he’d been up to, but I didn’t tell him how I’d found out. I caught him off guard, he put his foot in it and eventually admitted it. He had the cheek to tell me it was none of my business as I was “just a friend.” I didn’t respond. I haven’t spoken to him or seen him since but I knew he was rattled when he blocked me on Facebook.
I decided to tell his partner all about our so-called friendship: she deserved to know. I didn’t want to tell her straight away because I was furious: I needed to wait until I’d calmed down. I also knew that if I waited for a few weeks before telling her it would give him a false sense of security as he would think he’d got away with his awful lies and would dig himself into an even bigger hole by telling her some more.
I wanted to tell her what had gone on because I’d been in her shoes once before and wished someone had said something to me. I also wanted to hit him where it hurt by exposing him. He was under the impression that I was “too nice” (his words) so dropping a bomb on him would’ve come as a rude awakening.
I waited until Christmas was over and a few days into the new year I sent his partner a message on Facebook. My message was calm, factual and measured. I didn’t write anything salacious but she would have been left in no doubt as to what he’d been up to.
He wasn’t to know that I’d kept every text, WhatsApp and Facebook message he’d ever sent to me. I sent screenshots of some of them as evidence because I suspected he’d try to lie his way out of the situation and I wasn’t going to make it easy for him.
I made it clear to her that I was no threat to her as I didn’t want to see or hear from him again. I also said that I hoped his blood pressure would improve now he had one less woman in his harem (i.e. me).
She read my message in the morning and blocked me on FB later that evening. Oh to have been a fly on the wall! I don’t regret telling her. Any hurt she may have felt was caused by his antics. I suspect he didn’t think there was any harm in carrying on with two women because he didn’t think he’d get found out. He once asked me if I’d cheat on someone if I thought I could get away with it. I said I wouldn’t because the truth has a fascinating way of coming out in the end. You won’t be surprised to hear that he said he would. I suspect he’d never been faithful during the forty or so years of his marriage.
They’re still together. Someone told me she watches him like a hawk. I’m very grateful to be rid of him.
You hold all the cards OP. Play them wisely. Some people will advise you to rise above it, take the high road and be the bigger person. That’s all well and good if you’re comfortable feeling like a victim.