tired of feeling like a teeter totter

I turned 40 this past year and now I am having problems taking weight off. I am one of those people that lose weight only to turn around and put it back on again, it is a vicious cycle. After gaining 15lbs over about a 4-month time span I tried to start losing again about a month ago and somehow ended up gaining 3lbs, ouch. This time around something is different and I can't seem to lose weight despite eating reasonably healthy and exercising moderately. So I decided I am not going to diet this time instead I am going to revamp my entire thought process and make a lifestyle change. I am hoping My Fitness Pal will help give me the support to be more accountable for what I am eating and push me to exercise a little more. This time my goal is to take it off and keep it off! I would like to lose 20-25lbs by May (going on a cruise) and an addtional 5-10lbs after that.