How wonderful that you want to get your kitty to a healthy weight. This reduce her risk of diabetes and heart disease to name two diseases obese cats get.
You have to get her off the dry food.
Dry food is so biologically inappropriate for a cat to eat, you are never going to get her to a healthy weight feeding her some corn laded high carb junk from the vet.
What your cat needs is a good quality, low carbohydrate canned diet. (or a raw diet) Something that contains identified meat protein, no grains, little or no fruits and veggies.
Avoid "chunks in gravy" style foods because these are high in carbohydrates.
Get a baby scale and weigh her weekly to monitor progress. Cats should lose weight very slowly. No more than 3 or 4 ounces a week.
You will need to increase her exercise too, but that will probably be slow going until she loses enough weight to feel like moving. I'll be happy to offer some tips on that, if you come back to this thread.
Start the food transition slowly. Once you have her on an all canned diet you can judge how much she should be eating for safe weight loss, by weighing her weekly.
I'll be happy ton answer any questions.