Why would a guy cheat on every girlfriend he’s had with the same girl each time - but lies to the girl he cheats with?

My boyfriend who I live with has been talking to this girl who he has years of history with behind my back for the duration of our 3 year relationship!!!
They’ve been hooking up since they were in high school but haven’t seen each other in a few years.

He’s cheated on every girlfriend he’s had with this same girl but they’ve never actually been in a relationship with each other. He has messed her about for years but always goes back to her for some reason?

I have just found out he had arranged to cheat on me with her. Turns out he had lied to her too about his relationship with me.

He told me about her because I saw her name pop up on his phone because he used to text her all the time when he was drunk and they were best friends on snapchat with the stupid 2 pink hearts emoji...
I found out he told her he was planning on visiting her, wanted to be with her, fuck her and how he missed her so much and was he was texting her asking for naked photos etc... for months?

She was upset telling me all this too as he told her he was single and she told me he cheated on his last 2 girlfriends with her too... one when he was 19 and then again when he was 23. He’s 29 now and still going back to her and lying to her but never been in a relationship with her? Like why!!

Why does he keep going back to this girl?

Also to add she moved away to another country and they haven’t seen each other in like 4 years... why does he still want her and not me?

I don’t know what to do.
I asked him to block her on everything and he did!!!
Why would a guy cheat on every girlfriend he’s had with the same girl each time - but lies to the girl he cheats with?
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