The girl I have loved for 17 years is getting married to another dude. How do I stop the wedding?

OK long story short, I'm 28 and I've been in love with this girl since I was 11 (she was my friend's little sister). We used to be together, but five years ago I fell off my game and lapsed into loserdom for a few years. We broke up and she got with someone else during that time. I eventually got my sh*t together and resolved to get her back when her current relationship failed, but I just found out they're getting married in August so it's now or never. How do I stop the wedding?