What did it for me was doing deep dives into some 'hard' areas: wrote a parser and interpreter for a BASIC dialect in C, implemented 3d software rendering in 4k

What did it for me was doing deep dives into some 'hard' areas: wrote a parser and interpreter for a BASIC dialect in C, implemented 3d software rendering in 4k of Java, wrote some multithreaded client servers in C and Java, did some homebrew gameboy/gameboy color dev in z80 assembler. Workign through things like this really gives you a lot of confidence. There are a lot of resources online for this stuff nowadays. If there's something you read about that makes you go 'wow, that's impressive', define a manageable subset of the subject, and go do it!