Don and Marilyn split because they lost their child, Byron. Marilyn was unable to cope with her grief and with Summer Bay reminding her of the life she had had with him, she ran away. When we saw her again in the London episodes a couple of years later, there was no sign of the happy, bubbly, carefree Marilyn that we all know. She was still very much grieving. Don didn't find her insufferable or unbearable. He never stopped loving her and told her this. He asked her if she had stopped loving him and she said she had because it hurt to love, which sounded like she had given up on ever loving someone again, which is so sad.