I'm 71 years old and my brain is as sharp as it ever was, and I've had tinnitus for over 2 decades now. I wasn't aware there was a link between tinnitus and cognitive function. In the beginning, sure, I was always seemingly distracted, often suicidal and very, very depressed, but there is a lengthy period of adjustment and getting used to tinnitus, if that is the right term. Life goes on, and we eventually find a few ways that sorta, kinda help us get to sleep and function "normally".
It's just a part of my life now, and has been for a long, long time. Staying busy, getting plenty of exercise, eating right w/ lots of healthy food, finding a social group that supports us, having a meaningful job or hobby, these all are necessary for having a good life. They also help w/ our tinnitus. I'm committed to living the best life I can because this is the only life I will ever have. Mistakes will surely happen, life will happen, death to loved ones happens, but you just keep moving. Every morning I wake up is a gift, and an opportunity to fix what I may have screwed up today. The clock resets when the sun comes up.
I was once on corporate boards, and was also homeless for several years due to drug addiction. Being roughly awakened in dangerous homeless shelters at 6 AM, hitting the streets penniless all day rain or shine until we could go back in at 9 PM. I've owned condos in Hawaii, and slept on the side of the road in Reno, NV, only to be awakened w/ snow and ice on the blanket I had covering me. We never know what life will throw at us, you just make the best of it and keep on, keeping on.