Hopefully your teacher can help you work towards achieving these notes, and also keep up your enjoyment of playing by working on other things, so that they don't become too much of a "thing". One of these days, you may well find that they just happen, you won't know what's changed, but suddenly you'll be able to play them and you won't look back.
I agree - it's so important to keep some time for just playing what you enjoy; after all, isn't that why you are learning flute? There have been plenty of things I have found really hard along the way on my flute journey - double tonguing, triple tonguing, vibrato - and top F when I was at that standard!!
I'm so glad you've decided to "plod on". Please don't compare yourself to your cousin who had no regrets. From what you have said in this thread, I think you might have regrets if you stopped now. You are absolutely right in saying that you cannot do everything and that too much stress will affect your health. So why not cut yourself some slack while your job is intensive? Shorter practice sessions can be just as beneficial, but I think you need to put back some enjoyment into them instead of beating yourself up over what you are finding difficult. Might your teacher play some duets with you? That's something my teacher suggested at times when I've been finding things tough or life is getting on top of me. It helps me to remember why I started this journey.