Why is the weight coming off so slowly?

i weight 228 pounds and im 5'7 and im 27 years old. i started my diet like 3 weeks ago and i have done so good i dont eat anymore junk food, no more soda, i eat cheerios for breakfast a banana for snack, i drink alot of green tea, and at lunch i have a eas protien shake, for dinner its boneless chicken and green beans no er. i jog for about an hour 4-5 times a week. its been 3 weeks and i have only lost 8 pounds. is this normal or am i doing something wrong? i know losing it slowly is better and helps keep the weight off, but their are times when i dont lose a pound and times when i gain 1 or 2 back, but how? some one please help cuz im getting frustrated and i dont want to quit but its so hard. thank you