What a great question!From your original post, when you allude to "catching the tune", it sounds like you are sort of attempting to learn it by ear? Yes, you might have the score in front of you but it seems like your method of learning the song initially is to sort of get a feel for how the melody goes?Try sight-reading the piece. Perhaps just start with one hand, and go extra slowly. Do you have any success with that?Learning a new song is all about breaking it down into manageable bits and using whatever "tools" you have in your arsenal for conquering each of those bits.If your goal is to play the entire melodic section at once, so you "know how it goes", it's not surprising that you may find the piece challenging. This doesn't mean, however, that it is too hard for you!Once you confirm that your strategy for tackling the piece is the correct one, you'll have a good indication whether or not it's more difficult than most pieces you are used to!