Hayman told Dylan Cleaver's The Bounce that he's been struggling with head related issues since retiring in 2015.
"I spent several years thinking I was going crazy. At one stage that's genuinely what I thought. It was the constant headaches and all these things going on that I couldn't understand," Hayman said.
Those thoughts and symptoms led to Hayman abusing alcohol, having suicidal thoughts.
He ended up receiving a suspended prison sentence in France after admitting to charges of domestic violence.
Hayman, who now lives in Taranaki, told The Bounce he's joined 150 other players, including former England international Steve Thompson, who are suing World Rugby for failing to protect players from the risks caused by concussions and sub-concussions.
Thompson has said he no memories from the 2003 World Cup in Australia and claimed he suffers from memory lapses, anxiety and panic attacks.
Hayman said, "I'm 41, I've still got a massive part of my life ahead of me and when you live with something like this it certainly makes every day a challenge."
"The headaches were the start, and they were something that kept getting worse over time. Waking up daily with a constant headache at various levels that never really goes away.
"I started having substantial memory issues. I was trying to get a passport for my son and I couldn't remember his middle name, which was a significant moment. I was searching around for it in my mind for a good 25 seconds and had to go, 'I'm really sorry, I've forgotten', to the person on the phone trying to do the passport. 'I've forgotten my son's name'.
"I had temper issues, definitely, and then at this point of my life, it led down the track to what I'd consider alcohol abuse. I always enjoyed a beer with the boys but at this point I began drinking more. I didn't know what was going on and the drinking brought a little bit of an escape for a certain amount of time. It would temporarily alleviate the symptoms somewhat but then, as you can imagine, the next day things would be back to how they felt before, if not worse. It was a vicious cycle I got caught in."
"As time has gone on it has become more apparent what the symptoms are that are related to the CTE injuries: memory loss, anxiety, anger, depression and alcohol abuse. Recently I have had head spins, and get tongue-tied and find it difficult to find the right words in the first place."