If you were the other woman (unknowingly), are you allowed to say "you were cheated on"?

Last night, during a girls' night in, the conversation turned to exes. I shared my story too.

For context, I found out last November that my ex had a girlfriend of eight years the entire time we were together(6 months). She reached out to me, and it was a mess. I broke things off with him immediately and apologized to her. (She’s still with him and thinks I was chasing her man.)

The pain of that betrayal hasn’t fully faded, but last night, I was hit with something new. Two of my three friends told me, “You didn’t get cheated on. You were just the other woman.”

And the more they explained, the more it stung. They said, “You weren’t in a real relationship—you thought you were, but it was a lie. You might feel betrayed, but it’s not cheating when you were the side chick.”

Since then, I can’t stop thinking:

What am I even grieving? Does this pain even matter if I wasn’t really in the story? How did I not see the signs? Was I really that naïve? And, worst of all, is it my fault she was cheated on?

I’ve been crying for hours, questioning everything. Can someone please tell me—if you unknowingly become the other woman, are you even allowed to grieve the loss of what you thought was a real relationship? Or am I being unfair to the actual girlfriend?