seems like you are doing well enough. I’ve been studying for two years and consistently say things wrong as well despite talking in japanese literally every day. it’s going to take a while to fully understand things.
Some advice I have, and something I use up and through this day is to write down the grammar points in an explanitory way. Like explain them to people, maybe on italki blog about how to use a grammar point, similar to bunpro’s explaination. Of course, no plagairism, but putting it in your own words-summarization- is how to really cement a point into your head.
As for asking your gf for advice, to counter my friend up there, once you have sufficient kowledge in n4/n5 generally the others translate into something similar. so if you understand a more basic point or word, she should be able to help. ofcourse, do not ask 50 questions in a row or anything. she ain’t your tutor.
Another thing, I find childrens books harder than real books. I’m currently reading through 恐怖コレクター and it’s really good with n4 knowledge. I also find english translated books, like harry potter or CS lewis’s Lion witch and wardrobe series to be realtively easy reads as well, compared to children’s books.