Motivation and discipline are by far the hardest aspects. Especially if you aren't living in a country where you're target language is spoken. I'm currently trying to improve my Spanish while living in Hong Kong. There are plenty of native Spanish speakers here but their English is always so good, speaking in Spanish just slows down any natural social conversation. Not ideal when they are interested in socialising rather than doing impromptu language exchanges on nights out!
For me the hardest part about learning a language is to focus on your goal and have enough discipline to practice your language learning every day. Besides that point of view I think grammar is huge tough.
I feel like the hardest part of learning any language is pronunciation. People can memorize vocabulary and eventually get the grammar, but pronunciation always seems to hinder people the most.
i findly hard to focus on learning language. When i don't pay attention for what i studied i lost in continuing to the next level.