It’s so sad you’ve had to go through life only knowing those fifteen words

for languages you can already competently speak, reading is probably the best way to improve your vocab

As a guy in sorta the same situation (I suck at remembering shit), as soon as you memorize the very basics out of the way you can just learn the rest using immersion.

Thanks! Ironic that I didn't include spelling as one of the skills in this meme...

First: Find easier comprehensible input material. The whole point is to learn new words by guessing their meaning based on context. It aint gonna work if you don't know enough words to even understand the context. So, content meant for natives will be quite far along your journey. Start first with comprehensible input content meant for learners. And if even that is too difficult, go back to studying the words and grammatical structures that your textbook tells you to.

Second: At least in my experience, there's no easy way around doing a shitton of flashcard/sentence mining. Set aside time in your weekly (preferably daily) routine to just spam the fuck out of Anki / q*izlet / whatever other app you use and commit to it religiously. Practice makes perfect. Don't slack off and tell yourself that you're "not motivated". Force yourself to study even if you don't want to. Motivation is a finite resource, Discipline is renewable. Soon enough, you will find that it gets easier and easier to stay motivated. That is the essence of discipline. And, as a bonus, discipline is a universal skill: if you get better at motivating yourself in the context of learning a new language, you will also notice improvements in other areas of your life. Many people start learning a foreign language specifically to train their discipline.

Third: Only do n+1 sentence mining. That means, only make flash cards out of sentences that have only one word or construct that you don't already know. And if most of the sentences you encounter have more than that, it's a sign that you have to take a step back and learn the basic vocab sets that your textbook gives you before moving on to comprehensible input and sentence mining.

Fourth: As Stephen Covey said, take time to sharpen the saw. That means put some effort into researching different learning techniques that work for you. Otherwise, you'll just be wasting time on ineffective strategies. The previous advice I gave may not apply to you specifically, so it's best to do your own research.

I found it didn't work for me until i was almost A2.