I say this to you with the utmost love and respect...until you understand the reasons for your relationship with food (whether eating too much, too little, binging or purging) you cannot begin to change that behavior. That's why yo-yo dieting is the norm.
My advice, before dieting or attempting to change your eating habits, is to first seek counseling to understand your relationship with food and what underlies your eating more than you need to sustain a healthy life. Then you're ready to proceed to success. If Weight Watchers includes this counseling, then go for it! But do include this component in your new adventure toward wellness...
Just as a tree, though cut down,
sprouts up again if its roots remain uncut and firm,
even so, until the craving that lies dormant is rooted out,
suffering springs up again and again.
--from the writings of Buddha