I learnt Russian at uni and our course book was the New Penguin course. I found the order of things being taught weird but otherwise an excellent book. I had however learned German and French well, so was already linguistically oriented. Things like cases just make sense to me. I'm not sure how good the book is if you're not that way inclined.

I can only tell you, if you don't deeply understand what cases do, when they are used, and what other languages do without them, you will always struggle. So maybe worth reading up on english-language articles, books, wikipedia (not necessarily about Russian, maybe even German), to understand more about them.