I know what you mean. Learning Spanish vocabulary is much more of a challenge now that I'm old. (I'm amazed at all of the German vocabulary I still know even though I haven't used German much in the last 20 years.) 1. Reading helps. You not only find new, useful vocabulary, but you can run across previously learned (and perhaps forgotten) words as well. I avoid learning just lists of related words (like all of the words for building materials or political institutions). I need to see the word in context and then I choose those from that area that are useful. If you read an entire book, you'll see that a lot of the vocabulary is repeated throughout, so you can have vocabulary reinforced that way. 2. You can try some kind of SRS (Spaced Repetition Software). It's a computer-based flash card system that presents words based on how well you think you remember them from one review session to the next. If you don't know a word or had trouble thinking of it, it will be presented again the next day. If you know it well, it comes back in the review about 7 days later, then 21 days then 1.5 months, etc. I've been using Anki (but there are others). 3. You can write more so that you use the vocabulary. Use the words in situations that fit your real life so that you can relate it to something. You might even try to do a vocabulary journal. Write the word's "real" definition and then write the word's meaning in your life. For example, you can write down the standard, dictionary definition for "anxiety". Then write about anxiety in your life. Are you anxious? Do you know anyone who suffers from severe anxiety? Why do people have test anxiety? etc.

practice practice practice

just one way: use them everyday, for instance, write an essay use the words you should remembered.

As you already said to yourself, you try. And you try hard. And you try hard to remember and practice. They say in average you use a word for 70 times and you will passably learn it.

use that words in a sentences, and saved it in your brain.