Plato's Republic is the real deal. It's going to be a challenge—it's a challenge for everybody.
Since the Republic is primarily a work of philosophy, there isn't much of a conventional plot to move you along. All you have is the abstract thought-processes of Socrates to keep you on track. And Socrates may be a total genius, but that doesn't mean that he's always clear or straightforward. Lots of abstract concepts get mentioned without being defined, and shifts between big universals and the little, itty-bitty details can happen suddenly to confuse us.
But keep in mind that even people who have been reading Plato for years find him difficult, so difficulty shouldn't be a reason not to give the Republic a go. Remember that what's important about the Republic isn't just all the complicated proofs of this and that, it's the process of self-discovery and self-knowledge that's most important, and that is totally something you can get from reading this, even if you don't understand every single thing.
And hey, you've got Shmoop here at your back when things get tough.