I might be jumping to conclusions, but given that you’re talking about individual letters here…
…that’s more of a precondition than a first step really. So if you are still using mnemonics for the letters, then maybe take a weekend to get down the alphabet first. There’s just too much going on in your head otherwise if you’re trying to learn vocabulary whilst still learning your ABCs so to speak.
What exactly is it that makes the personal pronouns difficult for you? It really shouldn’t be the fact that…
…I mean that’s always true if you look at different cases for m/f/(n) pronouns in Germanic, Romance, or Slavic languages. In that sense Russian is no harder than any other foreign language you could learn. Coming back to the above point of, is it maybe that you still need more work on the alphabet itself, so that you can concentrate on what you really want to learn.
Given that you know German, it really shouldn’t be a problem with understanding the grammatical concept here, even though Russian has two more cases. You’re in a much better position than having English or French as your native language.
I think this is a good point for you to reflect upon and see what it is you’re trying to learn here. Latin is a dead language and in school we teach it more similar to a science class than to a living language class. So, you can obviously find tv shows and movies in Russian (which is not true for Latin) and learn it in a more natural way.
Just like in German, not all cases are equally likely to occur and just by listening to tv shows, even if you don’t understand much yet you’ll develop much more of a “natural ear” for the important cases. Also, great is the enemy of good, so if you just want to learn the language to communicate and not pass a test in school, think about how much trouble German speakers have with their cases:
Das ist Peters Auto.
Das ist Peter sein Auto.
Das ist das Auto von Peter.
Obviously, they’re all the same sentence, but grammatically only the first one is really correct. This is true for speakers of many language, so I’d really go 80/20 here and once you got the basics down start relying on listening to spoken Russian on tv or what you can read in magazines; rather than spending time with a grammar book.
Bottom line, make sure you got the alphabet down first. Understand the grammatical concept by ways of the table you’ve linked to but learn the language in context rather than a grammar table or word endings, which is also a point that Daniel already made…
…so listen, listen, listen. Tv shows, movies, YouTube, etc… people actually speak this language without thinking about the grammar all day. Just listen a lot until you can also say that “this sounds correct” rather than “this is the rule for this case”.