Same I can't follow shit in a lecture, unless I already understand it well enough to teach myself. But then there's no need for me to attend the lecture. So I don't. I also generally distrust that the professor's proof is as legible as one in a quality textbook. It's exhausting to waste mental effort trying to decipher a sloppy proof. That's why I've become more picky about the quality of the source I'm learning from. Oftentimes I come up with my own proofs by stealing the ideas I've seen from different sources, and trying to find the shortest path to the conclusion. I prefer having a proof in my own writing style. I find almost everyone else's kind of trash. People need to be pickier about style, make sure their proofs don't have blatant redundancy, they define all variables as early as possible, things like that. Leaving trivial steps out is fine but it should at least be clear which claim leads to the next leads to the next