One of the most difficult things when learning a language is to memorize vocabulary. Many times we trust our memory to remember all these millions of new words in another language, that we want to be able to control. But although we can rely on our memory we should not let our memory be the only tool used to memorize. So, we can find so many ways to help our memory. For example:
Focus on useful words: if these words you are studying or memorizing are not words that you are going to use constantly, they are rare words or words little used in your day to day routine; then you will tend to forget these words and as they are not important to you, you will not repeat them very often, so you will not be able to memorize them.
Put the words in context: you can learn a lot of vocabulary. It is normal that in your learning process you find yourself with a lot, a lot of vocabulary, but remember that these words have to make sense to you. So give them a context for you, so you can remember them more easily.
Find good sources of vocabulary: movies that are interesting for you, programs, series, podcasts, books, etc., all these are great source of interesting vocabulary. As long as these vocabulary sources are interesting for you, because if they are interesting to you, you can believe me that every word you read or hear you will never forget.
Create a map in your mind: where you can match the words that are related. If you match some words you have learned with other words you are still learning; These words that already have a strong connection in your memory will allow these new words to adhere to your memory much more quickly.
Create stories: when you find words that are very difficult to remember, try to use them in a story. Many times creating funny and short fictional stories can help us remember certain words.