The former Blue Peter presenter said his wife visited his GP three times and complained of persistent headaches, night sweats, bruises and fatigue, but was fired each time.
When his symptoms worsened in November 2017 he was told they were related to stress, he added.
After being rushed to A&E the same month, Gemma was diagnosed with adult acute myeloid leukemia and died three days later, on November 24.
Simon, who is currently promoting a competition to help Blood Cancer UK, was left with a broken heart and is only raising his 12-year-old son Ethan.
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Both received therapy after the tragic death, and Ethan’s counselor appeared on television.
“We both benefited from having people to talk to, and we both spent two years working with our individual counselors,” Simon told The Sun.
“Ethan’s counselor was an amazing guy named Colin. He’s an actor and a counselor and we saw him in a McDonald’s ad, but last week he was watching the John Darwin drama Canoe Man and he was there.
“I showed it to Ethan the next morning and we both loved seeing him.
Simon has remarried after marrying lawyer Derrina Jebb last July.