For me, one of the most difficult phases of language learning (by myself or inn part-time language courses) is after 1 or 2 years, when I have some basic knowledge but, as you start to learn more and increase your vocabulary, a lot of words sound/look "the same", this was the case with both German and Russian.
I think this is because you are trying to put a lot of knowledge in your brain but it's much harder to remember words without context. For example, take words you hear every day, you make lots of connections when you hear that word said by a person you know, in a situation you remember, so on and so on.
Learning similarities between new words and words you already know helps a lot. For this reason, I'm looking for some kind of book or software that shows related words (with the same root, same origin, etc) given some word, for example:
You look for the word monetary, you get also money, moneyed, and so on.
I've been using etymological dictionaries for this and it helps me somewhat, but not as much as I wanted.