@george11 Mah man... I got you. I'm the best with advice, @abby.22 will tell ya.
1. You can go over to her fiancé's house and challenge him to a fistfight. If you beat him, she'll be head over heels for you again. (This one depends on the size of the guy.)
2. If #1 doesn't work, try consulting her in an adult-like matter. That you didn't know she had a fiancé and you felt as if you were being used as a side piece (believe me man, I know how that feels). (This one depends on how much she'll listen to you and understand.)
3. If the first two don't work, don't worry. All you need is a knife/gun, a shovel, the lack of knowledge that she knows, and take her fiancé out to a field. (This one depends on her awareness or his fear.)