My Probably FA Partner Of 7 Years Cheated On Me With An Ex, And I Can't Make Sense Of It

Please, I need some outside perspectives.

My ex (33F) and I (26M) were together for 7 years. The last 5 years were long distance, but we were planning to finally close the distance within the next 6–12 months.

A year ago she proposed to me. Six months ago she was talking about wanting children within two years. In May she introduced me to her strict, traditional father for the first time, and we even talked about engagement rings.

Then, seemingly out of nowhere, she suddenly said she no longer wanted children and wanted to break up. During the breakup she cried, said she loved me, said her heart was breaking, and that she "couldn't give me what I needed to be happy."

Post-breakup I discovered she had secretly been talking to an ex from 10 years ago throughout June and slept with him almost immediately after ending the relationship. A call we were supposed to have that evening was postponed until the next day. During that call she sounded completely emotionally detached. Her voice was monotone, flat and almost robotic compared to how she had been before.

She has since refused to discuss the relationship or consider reconciliation. She said she just wanted to forget everything and move on, while also telling me she had to "consider his feelings." I only know about the cheating because I cornered her with tracker evidence.

What I can't understand is this: from my perspective our relationship was loving, affectionate and stable. We had disagreements like any couple, but nothing major, no abuse, no constant fighting, no dead bedroom, no obvious signs that she was deeply unhappy. We had just been making plans for a future together.

Has anyone experienced something similar? How does someone go from talking about marriage and children to emotionally checking out, reconnecting with an ex, and ending a 7-year relationship so abruptly?