Hello...It seems like you have been fighting a long battle to lose weight. Some people tend to have a low basal metabolic rate which makes it easier to put on weight. It only means that you may have to put in some more effort to lose weight. Any weight loss program to be effective, has to be sustainanble over a long period of time. Briefly put crash dieting or spurts of months going to the gym will not help. It is a misconception that having a thyroid problem will cause you to keep gaining weight. Whats important is to check whether your thyroid function is normal on your current medication. So please do that first since irregular periods can also occur with hypo functioning of the thyroid. Next consult a qualified dietitian who has had success with people on weight loss programs. They may be able to chart out a plan that is appropriate for you. Third and most important, you have to make significant lifestyle changes if you want to lose weight and keep it from coming back on again. So other than what and when you eat its also important to get a good amount of exercise(30 minutes) daily on most days of the week, avoid addictions ,be optimistic about life and get at least 7 hours of sleep everyday.