As a beginner, how should I go about learning difficult concepts? - sh.itjust.works

I’m trying to learn programming and something I struggle with the most is trying to separate code mentally into chunks where I can think through the problem. I’m not really sure how to describe it other than when I read a function to determine what it does then go to the next part of the code I’ve already forgotten how the function transforms the data and I get stuck trying to figure out the solution. So instead I’ll often cludge something together just to make it work but I don’t feel like I made any progress. Has anybody else run into this issue where they struggle with abstracting code from text to mental instructions?