A few months ago I started using Anki to fill the massive hole in my knowledge of German vocabulary. I've since studied more than 1600 flashcards and it's helped me immensely. Anki will automatically space out repetitions of the cards for you depending how difficult or easy you found them. This is called spaced repetition and is an evidence based way to remember things in the long term. Just make sure to force yourself to do it every day.

https://apps.ankiweb.net/ It's free and open source

For German learners, here is the deck that's helped me the most: https://ankiweb.net/shared/info/653061995

4000 words may seem like a lot but it's worth it. Every word has an example sentence, audio sample, as well as plural forms if a noun and for the verbs some conjugated forms and simple past and past participle.

also it's good for remembering other shit too. med students use it religiously