In your OP you mention examples of feeling unwell. Write those down and take them in with you. I have a thyroid condition and, after feeling unwell for years due to being medicated but UNDER medicated and being ignored and fobbed off with 'your blood test results are within range', I finally went in with a written list of symptoms, including numbers as examples. This was thanks, in part, to having a tracker watch and a thermometer and being able to refer to the readings from them. I used these as headline figures then followed up with all the other symptoms I had but couldn't quantify. I also had written examples that really, really explained the impact each symptom was having on me and my daily life. That was the first time I was listened to.
For example, my body temperature is xyz, which is below what it should be (I believe the average is xyz) and I feel cold all the time. My resting heart rate is dropping to xyz, when I believe the average is xyz, and I'm always tired and struggling to wake up every morning making me late for work.
You MUST write it down and take it in with you, if it helps. If you feel you aren't being listened to, you need to be as prepared as possible to 'over'-advocate in order to push your point home. You deserve this. How many hours are you sleeping each night then on a scale of 1 to 10, how tired are you? What actually is your body temperature usually/over a couple of days when you wake up, at lunchtime, at bedtime? Note it down over a couple of days and take it in to the appointment with you.
Having a thyroid condition could itself be impacting on your ability to explain yourself as it affects how quickly your body 'lives', moves, thinks etc etc. Writing things down in your own time beforehand is probably useful to the GP as well as you. Put yourself in the GPs position here too. A list of specific symptoms and clear examples of the impact they are having is far easier to work with than a list of general symptoms that you sort of remember in the moment. The more specific you can be the better and, if you need to do that in writing, then so be it.
If they refuse to accept this, speak to the practice manager and ask for an appointment with a different GP. It's your right to be given appropriate healthcare.
Also, if you can, check the NICE guidelines for the conditions you suspect. They usually set out guidelines for the GP to follow when treating conditions.
(Sorry this response got quite long! Good luck OP. You can do this. xx)