I think adults get more frustrated than young learners when they seem unable to remember things. They probably have poorer short term memory than younger students. Even high school and university students probably have an ability to cram information into their heads for exams that adult learners have lost.
So I believe you should not rely on memory to learn languages. Expect to forget most of what you learn. That is why consistent and intensive exposure through listening and reading to meaningful content is so important. Through the process of ingesting the language, often the same content, over and over again, you gradually improve your ability to deal with the new language.
Of course it is also important to make the effort to learn the bits and pieces, the words and phrases. Flash cards or similar systems are very effective. Working on the words and seeing them again in different sentences that you have heard or listened to (as we do in The Linguist) is a good exercise for the brain. It also helps you learn how to use them.
But when you seem unable to remember these words and phrases, do not worry. You are still training the brain to process the new language. The effect of all of this is cumulative even if at times you feel you are just not improving. We all have a tendency to overestimate the possibilities of short term change and to underestimate the opportunities for long term change.