I’ve been learning German for about one year but found myself still struggling with certain grammatical errors. Some grammatical mistakes I made with writing always escaped my own proofread; however, a few days later when I read it again, I am always amazed how such mistakes went unnoticed. I’ve tried some websites, e.g. Rechtschreibprüfung, but found them not really useful. Certain simple errors still escaped the review by the websites. Is there any website for German anyone would recommend, that is at least to be as good as Grammarly?
We recommend https://languagetool.org 
But it’s true that LT isn’t optimized for German learners (at least not that much). But please send us examples of your “real errors” (ones that are not made up), so we can improve LT.