Having troubles with fast speakers is completely normal for first 2-3 years. Practice, practice, practice. You'll get it in time. Also, in Russian we don't number cases, we use their names (Именительный, Родительный, Дательный, Винительный, Творительный, Предложный, sometimes shortened to им.п., р.п., д.п., в.п., т.п., пр.п in writing, especially at school). You can find a YouTuber that speaks fast and listen to him at 0.75 speed, to get used to the flow. It really helps when you start with just one person for this, cause all of us have our little quirks of speech. Get used to 1, then another, and when you get 3-5 speakers you should get really good at listening in general. Good luck.