I Recently Found Out the Guy I’ve Been Dating Has a Girlfriend. What Should I Do?

I recently found out na may girlfriend pala yung guy I’ve been dating and involved with for a long time. I genuinely had no idea na he was in a relationship. I was blindsided with all the dates, gifts, gym time together and time he gives.

His IG makes him look very single—he barely posts anything—so I never really suspected anything. Then randomly, I searched for him on Facebook (we’re not even friends there) and saw a tagged photo from a family dinner last month. He was with a girl, and naka akbay siya sa girl throughout the photos. I later realized na girlfriend pala niya which I know kung sino din ung girl.

The worst part? We met a few days after that family dinner. 😐

Right now, I honestly feel so blindsided and disgusted with the whole situation. I also feel guilty because I’ve been in the girlfriend’s position before, so this is bringing back a lot of emotions and trauma for me. I keep thinking na she has no idea what’s happening, and I know how painful that can be.
I’m not planning to confront him or interrogate him right now because I’m honestly too overwhelmed. And at this point, I feel like there’s no explanation that would change the fact that he was seeing me while having a girlfriend.

My gut is telling me na eventually, I should tell her. I’m thinking of sending her a message with screenshots/evidence of our conversations so she can decide for herself. I might use a dummy account because I don’t want to get dragged into their relationship or open another conversation with him because kadiri. I just want her to know what situation she’s in, then leave it there. But I’m not doing anything right now. I need to process everything first because I’m honestly crashing out over this. 😭

Would you tell the girlfriend? Would you confront him first? Or would you just leave it alone and walk away?