Getting back to my old self

I am currently the heaviest I have ever been at 173 lbs. Throughout high school I was a competitive swimmer and regularly ate 3500 calories a day to maintain my 130 lb weight. When I went to college I continued to eat like I was still swimming 2+ miles a day but with almost no exercise I started to gain weight. I made it up to 160 lbs by the end of the first year but went on a crash diet that summer and lost 20 lbs. I managed to keep it off for the next 2 years but started gaining again when my fiance moved in with me. In the last year and a half I have gained all the weight back plus some. We are getting married in April (5 months!) and my dress no longer fits so I must loose some weight. Plus I just don't feel pretty as I am now. So that's my story... Wish me luck!