How do I lose my fat bum?

Nobody can. Keep doing what you are doing. That's probably not the advice that you wanted, but the principle to keep in mind is that one cannot spot reduce fat. Each gender stores their fat in their body differently. In you, it sounds like it mostly goes to your "bum". Fat doesn't "shift" from one place to the other. You just need to keep persevering with your weight loss goals so that you will start to see the results you are looking to achieve.