Pretending to be happy will not make you happy. Instead give yourself something to be happy about. Do 2 things one that will immediately make you feel happy like helping someone carry their groceries or suggesting a good movie to someone that's frazzled in blockbuster. By helping someone else even with something simple you will immediately feel much better. Longterm, try to figure out what is making you unhappy and change it. If you don't like your body change it, start small and think big. Also find a hobby that doesn't focus on your problem, if you think your overeating,start painting,listening to james brown, sewing, anything that doesn't involve food. Basically get to the root of your unhappiness and derail it!