Congrats on the engagement!
I lost 30 lbs before my wedding (the most successful I have ever been losing weight thus far). The thought of my huge stomach in a wedding dress disgusted me, and was also the best motivation!
It became must more difficult to lose the couple months prior to the wedding...I was so busy with wedding plans (and school), that I was only able to maintain my 30 lb loss and I didn't lose anymore.
Be careful after the wedding! I regained all of the weight, plus some, within a couple months. It was very difficult without the "having to look good in a wedding dress" motivation.
Good luck! Really, an upcoming wedding is great motivation.
Oh, and corset-back dresses are the best....I laced mine to the point of not breathing for ceremony/pictures, and let it out a little for the partying