I've been diagnosed with hypothyroidism. I follow a good diet and exercise and my metabolism has been checked and its fast. Hard to lose weight still?

Check bmr. With hypothyroidism and difficulty losing weight, have your T3 (liothyronine) checked since that can be a problem. Alsoyour resting metabolism rate can be measured in weight loss clinics that will explain what your true metabolism rate is and from that decrease your calories by 500cal/day to lose one pound per week. Drink water since that helps with weight loss.
Exercise. If thyroid functions are normal and calories are not to excess, perhaps the frequency of exercise needs to be increased.