I am a mediocre developer

Some great suggestions here. One thing I've found is that developers commonly don't take the time to set themselves tooling-wise. Even simple things like linting built into your editor can provide a big lift by helping catch certain classes of bugs. That's especially true in a dynamic language. But in my experience many people don't want to take the time for that sort of thing.

> Whenever you find something in the code you do not understand - it is too complex.

+100 to this. The general measure of whether code is good should be something like the median understanding of all the people working on the project. Something that makes a lot of sense to only one developer ultimately does not serve the group. I think people forget this quite a bit.