Hi Milly's Mummy.
Yes, it is very difficult to continually go through cycles with disappointment after disappointment. I think you will know when it's time to stop. As for myself, I am already thinking about quitting (currently 6th cycle without result). Really, I only have two pieces of advice. The first is to take a break if you're emotionally worn down. Take a couple of months off from doing cycles. Have a holiday, forget about it all, and when you come back you can see how you feel about going on or not. The second piece of advice is to cherish the children you do have, and be very very appreciative, because there are a lot of us here who will never know what it's like to have children.