I experienced the same problem as you. For years I would try to lose weight and get into shape but it would only last for a few weeks at best. Right now I've been on a successful change of life plan for 2+ months and I know I'm not quitting this time.
The thing that really changed it all for me was this website honestly. It was the support of people and the expectation to lose weight and get more in shape. I make sure to do an update thread every month just to remind people about whats going on with my diet. A good thing to do is just let everyone you know, know about your diet and how your attempting to get into shape. Hopefully you will have at least one friend that will keep asking you about it, for me that one friend was my fiance. Originally I wasn't telling her my plans but the people on these forums convinced me it would be the right thing to do, and now everyone in my life knows about my change of life style.
Also, set realistic goals. It's going to do you no good to set your initial goal to look like Daniel Craig or someone with a similar body. That takes tons of dedication and time to reach those results. Set small goals first. My first goal was to just get my diet on track for a month, after the first month I incorporated cardio, and now I'm incorporating weight lifting.
That brings up another thing that hinders most attempts. Don't just jump in full steam ahead and blindly. Know what you want to do and start out slowely. You don't want to go to the gym for 2 hours a day lifting weights and cardio and diet all at once. If you're having trouble, like I was, its best to take it one step out of a time. Get your diet in check first, this is the most important part of becoming healthy and in shape. You can lift weights and run all you want, but if you filling your face with all types of crap, you will be busting your ass off for nothing.
http://scoobysworkshop.com/calorie-calculator/ Click that link and find out how much you should be eating a day.
Another good thing to do is take a picture of you today, ever 2 months or so, take another picture. This way you will get to see progress, even if you don't see it when looking in the mirror. I couldn't tell my stomach was smaller till I took a side shot two months later and noticed it was smaller. Stuff like this are good motivation boosters.
Getting past the first month of your change in life is the hardest part. It takes a lot of dedication and will power, but you can do it man. This website and these forums are filled with tons of great information and people who will be friendly and give you excellent advice and guidance while you go through all of this.
