under the influence and went through rehab. >> it's very humbling and it's also humiliating. >> reporter: but his biggest battle was yet to come. in 2017, cassidy revealed that he suffered from dementia. his mother had died of complications from alzheimer's disease only a few years before. >> to watch someone that raised you and was so vibrant start to lose their mind and disappear is arguably the most painful thing i've ever experienced. >> looking back on his own life, there is one memory cassidy hopes will never fade. his 1972 concert in madison square garden. cassidy leaped on to the stage in his signature white sequin jumpsuit, thousands of adoring fans screamed his name. his own family among them. >> it was just so emotional for me. and i just felt so blessed to have that moment with them. i mean, it's the highlight of my