I met my dream woman a week after I got engaged

I gave my girlfriend of two and a half years an engagement ring and just a little over a week later I met the woman of my dreams who is single and interested in having a relationship with me. How do I break things off with my girl without breaking her heart? I'm convinced without any iota of doubt that this new girl is The One. What would you advise? Why did I get engaged when I wasn't sure? Pressure! Some guys know what I mean...