The only way to learn to play piano is to play it. Beginner students must go through the process of learning the notes, etc, which has to be a part of "piano learning". Of course. The sightreading and memorizing comes along in that process of "piano learning" as well.
Also, to learn a piece, 5 years seems like a long time to me. But mostly, for most students, it may take 3 to 6 months; it all depends upon the length and level of difficulty of the piece. If a piece takes too long to learn, then go work on another piece. I won't work on a piece for very long.