Losing weight

So I was diagnosed with Hashimoto’s and thyroid cancer when I was 15. I had two surgeries and radiation and finally I am in the clear. Since then I have gained well over 70 pounds and I am struggling to get it off. I have tried diets but they make my moods awful and I feel sick all the time. I have no idea what to do, I’ve even been going to the gym multiple days a week and pushing myself to the point of almost vomiting. I just really am looking for ways to lose weight and be healthy again.