Hi dispenser,
I'm afraid with pets, the simple fact that that if more calories go in than come out, the animal will be overweight.
How long has she been on the diet you have described, and has she lost any weight with it at all?
If your cat gets nothing else to eat other than what you are saying (i.e. nothing else to eat of any kind, no treats, no human food e.g. gravy added to the cat food, and no milk), and she is still gaining weight, then she is still getting too much to eat, and I would suggest decreasing the amount of Felix you give. The weight loss diets from the vets are designed to have less energy and more volume, so they fill the animal up without giving it too many calories.
However, if she is losing weight, even if it is slow, I wouldn't change anything just yet. Trying to lose too much too quickly can result in even more problems for the cat.
Try encouraging her to play more and be more active. Maybe chasing games with bits of scrunched up paper or a laser pointer pen? I know it can be very hard with cats to try and get them to do anything other than sleep! Please persevere with the diet too, as a slimmer cat is a much healthier cat!