I suspect you're burnt out. Surely you can hold more than 2 levels of a callstack in your head at 40, so something else is going on.
Of course the code can also be particularly convoluted with some domain-specific knowledge that you may not necessarily be an expert on, so in that case, I'd find someone to ask questions or or an intro book until you can get to an 80/20 situation, where you feel like you understood most of the domain context.
If it's not the code, take a break. Long walks in a place with trees helps clear the mind. Longer sleep, exercise to get the blood flowing, no alcohol. All the usual healthy stuff.
You could also try to draw the data flow on a whiteboard, in a "follow the money" kind of way.
Alternatively, if time permits, try to reimplement the algorithm or solve the problem yourself, in your own preferred language/environment. This will get you sufficient knowledge about the functional requirements where you'll be able to see what the author of the code you're porting was thinking, since they likely encountered the same challenges as you.
If nothing else works, you can just be honest with your manager and try another project that feels like it would be easier or more fun.
Of course the code can also be particularly convoluted with some domain-specific knowledge that you may not necessarily be an expert on, so in that case, I'd find someone to ask questions or or an intro book until you can get to an 80/20 situation, where you feel like you understood most of the domain context.
If it's not the code, take a break. Long walks in a place with trees helps clear the mind. Longer sleep, exercise to get the blood flowing, no alcohol. All the usual healthy stuff.
You could also try to draw the data flow on a whiteboard, in a "follow the money" kind of way.
Alternatively, if time permits, try to reimplement the algorithm or solve the problem yourself, in your own preferred language/environment. This will get you sufficient knowledge about the functional requirements where you'll be able to see what the author of the code you're porting was thinking, since they likely encountered the same challenges as you.
If nothing else works, you can just be honest with your manager and try another project that feels like it would be easier or more fun.
Good luck!