Cognitive decline

Has anyone struggled with the feeling of not being nowhere close as sharp as you once were? I’m not Einstein but I like to think I used to be relatively smart and always did really well academically and all that 

I did 3y of banking at a real sweatshop and in the past year moved to a PE role. When I was in banking the work was easier and not having enough sleep was always a good excuse for not feeling really sharp. Now in PE I find myself having to deal with understanding a lot more things to a greater depth (commercial / financial / tax /legal / portfolio work etc) and it just feels like I hit all the time a real mental ceiling and that I am not really absorbing what I should be absorbing and constantly need to repeat basic concepts / have brain fog / memory issues. I feel really slow mentally especially while listening on calls, and I struggle to visualize things mentally (which I used to be really good at and always had a great visual memory). Reviews are all great but I feel like this will come out the more I progress and I need to own more things / drive sessions / build on accumulated knowledge (which I feel like I am not accumulating) 

Has anybody been through something similar or has any tips? I’ll go see a doctor but just wanted to hear if anybody went through something similar / whether banking lack of sleep has really messed up anybody else