So, I'm not the type to give people a morality lesson when they haven't asked for one because I think it's rude, but something you said stuck out. You said you weren't in love with her until a few weeks after you were apart? But you were already engaged? I don't understand -- how could you possibly be engaged to someone that you weren't in love with? I was just confused on that.
Anyhow, "free will" is a joke to me, and there are plenty of things that you can do to encourage your love to grow. But you can't change someone's nature -- if she's inclined to cheat on you, she's going to probably do it again.
Some things you can do: a honey jar with hot ingredients, like cinnamon or ginger, to heat up the love, and a petition for your love to grow stronger.
A lodestone spell in which you name each stone for each party and draw them closer every night alongside some candles and prayer. Eventually, after 3, 7, or 9 days you'll keep them together on your altar until you want to dispose of them. Lodestones are powerful, powerful things.
Buy some sachet powders and spread them around where she lives, making sure she walks through them. Fire of Love, Love Me, or Stay With Me all might work, as well as Follow Me Girl or even, since you're engaged, the marriage ones. Write her nice notes and dress them with those powders, too.
Even just a simple candle spell, grab a 7-day candle, a photo of the two of you, and pray over it daily while it's lit to encourage her to love you, honor you, and stay with you.