The single most important fact in feline weight loss: it must happen slowly. When an overweight cat loses weight too fast - or stops eating altogether - fat mobilized to the liver can overwhelm it, causing hepatic lipidosis, a genuinely life-threatening condition.
That's why the crash-diet instinct that merely fails with humans is dangerous with cats, and why any plan that has your cat refusing food entirely is an emergency to correct, not perseverance to admire. Gradual is not the cautious version of the plan; it is the plan.