Montana Brown broke down in tears during an interview on This Morning as she discussed her final contact with Love Island co-star Mike Thalassitis.
The 26-year-old reality TV star was found dead in a north London park on Saturday.
Police have confirmed they are not treating his death as suspicious.
Brown, who starred in the 2017 series of the ITV2 dating series alongside him, said she had the blessing of his family to appear on the programme because she “wanted to respect their wishes in coming here today and speaking about it”.
She said: “They’re as shocked as I am because I think they feel the same in the sense that, at the end of last year, potentially we could have been like, ‘ah I could have seen that coming’.
“But now, he’d been booking trips, he booked a trip to California with his best friend for a couple of weeks, he wanted to go to Sweden, he was about to open his cafe, he’s been going to the gym.
“So I think that’s why I really struggled because, in my eyes, there was just no reason for him to do that and I don’t know if it was just a spur of the moment, spontaneous decision where he had a moment of weakness but it makes me so sad that he obviously just had so many demons that he went through on his own.”
Brown said she had last spoken to him “two days before” he was found dead.
“I’m kind of kicking myself because he texted me and I didn’t actually reply, just because I’m really bad on my phone,” she said.
“You just overthink everything, what do you, and I’d just wished I’d replied.”
She said Thalassitis had messaged her, asking how she was doing, and she said she had not replied to him.
Thalassitis, a reality TV star and semi-professional footballer, found fame on the competitive dating show nearly two years ago.
Mental illness is a serious disease. I’m cooped up in my bed, just trying to fathom why Mike did this? Why he didn’t say something. Why he thought this was his only way out. I really am so confused. I feel so sad, I’m so sorry darling I didn’t do more. Miss you so much 💙
— MONTANA ROSE BROWN (@MontanaRoseB) March 17, 2019
He earned the nickname Muggy Mike after partnering with fellow islander Chris Hughes’s girlfriend Olivia Attwood.
– Samaritans is available 24/7 every day of the year, to listen and offer support to anyone who is struggling to cope. People can contact Samaritans by phone, free of charge, on 116123, via email at jo@samaritans.org or can visit www.samaritans.org to find details of their local branch.