The doctors should be the ones narrowing it down, not you. Please keep trying and maybe try different doctors if this is possible. It took 4 doctors for my symptoms to be taken seriously enough to investigate. The first step would most likely be a full blood test. Deficiencies in things like iron, Vitamin B12, and Vitamin D can have symptoms of fatigue, breathlessness, tiredness, palpatations etc. If these came back fine they could look into other conditions such as POTS or CFS/ME, which also share symptoms with what you've described. I'm not saying that there's a 100% you have any of these, that's for the medical professionals to figure out, but since they're asking you for possibilities then this may help. I'd research into them and then go to your next appointment prepared with conditions that share your symptoms. Please note that this knowledge is from my personal experience and I'm not a medical professional.