Why am I having such a difficult time losing weight?

I (28F) am 246lbs. I have a congenital heart defect (lqts type 2). I have been on propranolol for the past 10 years and 9 months. I don’t know what dosage I was on in the beginning but for at least 7 years I have been taking 120mg propranolol er. I’ve been struggling with weight loss for the past 3 years. I’ve tried personal trainers, meal replacements, diets, working out 3+ times per week, intermittent fasting, etc. you name it, I’ve probably tried it. 4 years ago, I was 180lbs. At my heaviest in the past six months, I was 255lbs. My cardiologist doesn’t seem to be concerned about my weight gain. My bloodwork perfect. However, my QT range has suddenly been dropping too low then spiking too high every other 6 month visit. I constant lose the same 10+ pounds over and over again. 255-242lbs. I can’t cut anymore calories because I’m literally exhausted by 4pm every day. Every time I talk about this with my pc, they check my thyroid function, results are normal. Why is this happening? What steps should I take to get my cardiologist to listen to me? Please advise me.