Don't feel bad, writing a game engine is one of the most ambitious projects you can do! As it involves many interdisciplinary skills; GUI, Graphics rendering, State management, Physics simulation, Coordinate systems, Sound systems, AI.

For writing your own OS, I definitely recommend looking at https://www.nand2tetris.org/course it gives you a holistic view of a computer and it's operating system at every level of detail.

I’d recommend spending a few weeks this summer working through the nandtotetris course it was something I only stumbled upon after college and it helped glue all of the different classes together in a simple cognitively digestible example.