I found out the guy I was exclusively dating was seeing multiple girls. Should I tell the potential other woman he is seeing?

A year ago, I met a guy on Hinge. It quickly became long-distance but it became more intense throughout the year despite not meeting physically. There was always one girl he followed I'd worried about. I told him that I'm uncomfortable if there are other girls. Our messages were becoming sexual so I set the boundary. He said there are no other girls, and he's not ok with me seeing other guys - i. e., exclusivity. I began to trade pictures (I never sent even half-nudes, but he tried to encourage it). However, his behaviour gave me doubts until a week ago I did some investigating. I easily found a sexually-toned 'New here' Hinge account, on top of my worries about the other girl.

I confronted him and he confessed. He thought our relationship wasn't serious and was 'keeping his options open' - yet admits that we're meant to be exclusive. He'd been sleeping with girl (s?), then returning home to chat with me. He is 'scared of committing' as he was cheated on before. I said he must at least tell me he doesn't have a girlfriend and he said, 'I wasn't serious, don't worry'.

However, a liar is a liar. I keep thinking about that one girl who I know about that might be intimately involved with him. So much evidence points towards a relationship - he goes to stay with her on weekends, he likes all her photos on multiple accounts, he's on her social media (she's never appeared on his), there was a story where they were hugging under the stars, he's met her friends and even went to one of their weddings with her... I drafted a message to send her but I'm scared. I'm afraid I got it wrong, they are friends or in a casual relationship, and I'm making a fool of myself even further. And if he finds out, he'll hate me. Unfortunately, it's still raw and I'm emotionally attached to him. But if she has expectations of a relationship with him like I did, she deserves to know.

Does anyone have any advice? I am scared either decision will eat away at me for the rest of my life.

I found out the guy I was exclusively dating was seeing multiple girls. Should I tell the potential other woman he is seeing?
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