This is one of those problems most experience. It is what we refer to as yo-yo dieting.

Bad thing is, the more you yo-yo the harder and harder it will be to get anything to work. Your body starts saying "oh not this again" and holds on to every fat cell it has and burns lean muscle tissue thereby slowing your metobolism even more. It doesn't help that aging has this effect as well. So the older we get and the more we yo-yo back and forth on a diet, the slower and more sluggish our metobolism gets.

Good news is, you can rebuild that metobolism, by building lean muscle tissue. Weight lifting. By building muscle we increase our metobolism thereby kicking in that fat burning mode. It does not however happen over night and it is a lot of work.

My advice would be, stop using Atkin's as a "diet" and start looking at it more as a way of life. Get use to the idea that regardless of how long it is going to take to lose the weight, you are and will continue to eat correctly and healthy. It has taken me 15 months to lose 50 pounds. It took me 10 years to gain 60 of it so I am WAY ahead of the game at this point. It comes off really slow but that is okay with me.