If she believed this, how come she kicked me loose and moved on so fast?

After two years of being broken up after dating a few months in highschool, and breaking up due to her religious beliefs, my ex reached out to me, claiming she was told by the holy ghost to reach out to me.
After a few weeks of catching up and talking about things, she says that she can't change her mind and we still can't date. At that point I admitted that I had been in love with her, and if we can't be together, then it would be too hard for me to stay friends.
She immediately stopped me, saying she didn't want to lose me again, and after a few weeks we were dating again. This went on for about 9 months, and she said she loved me back after 1.
She was in a different state for school, but we talked every day. But she had decided to go on a mission, and before she left she came and spent two weeks with me.
During the visit, and our time together, she said she deeply regretted ever letting me go. That I was her home, her best friend, and that she wanted to marry me. She said she didn't think we'd ever be on good terms again, and there we were, and she said she didn't think she'd find anyone who loved her like I did. She even almost cried when we parted ways, and she never cried over anything.
Anyway, she 100% changed on her mission. She shut me out and said we couldn't date anymore. I even wrote her two letters and tried to work things out. I put them on GaG and everyone said they were amazing and said how much I loved her. But she only cared about converting me. But even when I fought back, she said I knew her better than anyone and I knew her hear
Before she came back, I wrote one last letter, but she ignored me. I couldn't reach her.
Then I ran into her when she came back, and she lied to me about how she was feeling, and blamed the relationship not working on me. But all her reasons were from out time in highschool, and the religious stuff. She walked off and I only ran into her once since, and she stared at me from afar until I noticed.
But, now she's engaged.
If she believed this, how come she kicked me loose and moved on so fast?
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