It's the one part of your body that is very good at convincing the rest that they are too tired, that they've done enough, that nothing they do is working, that what really matters is the beauty that you possess inside.
Just a disclaimer: I'm not saying that you're not beautiful if you're not thin. That's missing the forest for the trees. Being thin isn't the point. Being beautiful isn't the point. Not being fat isn't the point either. The point is avoiding health risks.
Anyways, here are some things that you can do to convince the brain to work with the rest of your body, instead of being the proverbial wet blanket:
Know Your Limits, Master Bruce.
According to experts, many people exercise on a regular basis and have a healthy diet but overlook their levels of stress. Not only is this counter productive, it actually negates your efforts as stress can cause overeating, lack of motivation, and a marked increase in aging.
One of the best ways to avoid stress is to know your limits. Don't push yourself to the breaking point, both in terms of diet and exercise. You should also practice common sense. If this is your first time to go to the gym, don't try to work out like a professional bodybuilder. Start small and try to progress from there.
Food is Your Body's Fuel, Not its Drug
This is more of a reminder than a tip, but remember that food's purpose is to give you energy, not pleasure. This doesn't mean that you're going to force yourself to stop enjoying the taste of food, just that you'll have to accept that any enjoyment you get out of food is a bonus, not the purpose of eating. This mindset will also help you eat more healthy foods, as they usually turn many people off due to their taste not being as good as junk food's.
You Have Enough Time for Exercise
Stop making excuses. For an average person, 30 minutes of cardio every day should be enough. That's the amount of time it takes to watch a single TV show. It's also the amount of time it takes to get from one save point to another in some Role Playing Games. Do some brisk walking or jogging for thirty minutes a day. Don't have any place for it? Jog in place. Walk up and down the stairs. Be creative. In fact, if you have access to a treadmill or one of those hydraulic steppers, you don't need to sacrifice that TV time. Position your machine in front of the TV and burn some calories while watching your favorite TV show (although it should be noted that there are studies suggesting that your metabolism actually slows down while watching TV, so maybe you can read a book or listen to music instead)
Birds of the Same Feather
It is a fact that your environment can affect who you are and the people you are with can greatly influence your way of living. Therefore, it is best to be with fit and healthy people so that it will be easier for you to be more motivated and encouraged to live healthy. If you don’t have health-conscious friends, go find some. If you already have health-conscious friends, you should still go out and find some more.
Try to Set More Productive Goals
Are you familiar with the concept of a losing battle? Dieting and Exercising for the purpose of looking good in the eyes of other people is one of them. It's a losing battle because trying to impress other people subconsciously trains you into losing self-confidence, which results in you further believing that you're not good enough for other people and must do more work in order to keep up. It's the kind of downward spiral that will give Junji Ito nightmares.
If you really want your efforts to not go to waste, you should keep in mind that your goal is to feel healthy and prolong your life. Getting those washboard abs is just a bonus. Besides, if your main goal is to just be healthy and fit enough that you're not a ticking timebomb of heart attacks and cancer, it's going to be that much easier to be happy if your efforts are going slower than expected.