Well, not to be rude or anything, but if I personally want to use some fancy words, I just need to make it a priority to use them. You either use them or lose them.
There is a principle called the Linguistic economy principle. Its objective is to save more time and energy by conveying more information with less effort. It's natural to use simple words and structures. We apply this principle in our native language, too. Why bother saying "It ticks all the right boxes, it's absolutely unbeatable, it's second to none" if you can say "it's the best"? It basically means the same. It doesn't make much practical sense unless you're an ESL student aiming at expanding your vocabulary????
So, what do I do about it?
1. When revising the words, I make sure I am not just mechanically going through the word lists. Rereading notes isn't the most effective learning method. You have to practice retrieval: cover your notes and reconstruct them from your memory; actively recall words using tests and quizzes, which you can quickly create online. Notice how the words are used, try using the target language when talking about different things. Doing that regularly flattens the forgetting curve. I try to be fully present when memorizing the words.
2. Select 5-7 words I want to practice and make active use of before the learning session. Think about possible contexts in which I might use these words. Write them somewhere so I can see them and refer to them at all times. ⠀
3. Reflect back on whether I managed to use some of them during your learning session. If not, why? Where might it have gone wrong? Is there a way I can improve that next time?
4. Last but definitely not least, I take on personal responsibility for my learning progress. You have to own it. There is no way advanced words will appear in your speech unless you choose to use them