When I hear a cool piano song, I buy the sheet music and learn it. But after perfecting it for days, it sounds boring and it gets annoying to play. It gets less interesting, and doesn't sound as good as I would like because I played it so many times.
Is there a way to fix?
A couple of things spring to mind.
Get yourself so much better at sightreading that you can pick up the sheet music, and play it perfectly first time - not kidding - many players do this. That will alleviate the boredom that sets in when one has to play the same piece hudreds of times to perfect it.
Once you can play it really well, there are several options. You could extemporise over the chords, jazzing it up, if you like, making new melodies that fit with the chords. You could even incorporate that into your renditions of the piece.
You are probably aware that sheet music often isn't (and doesn't have to be) accurate to a recording. Find an original, and find where you could play more faithfully to that, using the sheet as a template, a start point.
You could play it in a different key - or even change major to minor, and vice versa. You could re-voice the accompaniment - same chords, but change the arrangement, even. Change some of the harmonies, you may even find ones you like better! The list goes on - and on!
You could try the piece in different tempos, different styles, different time signatures. That's a favourite of mine - I can't honestly say I know a piece inside out till I can play it in a different time signature - 4/4 into 3/4 or 5/4.
What is an example song, and the particular sheet music you've used?
Most pop song sheet music is arrangements for piano and not real piano parts. Those particular arrangements hold no special place as the only, or correct way to play "the song."
You can do your own "arrangement" of a song, either rehearsed or spontaneously.
With most songs the principle thing is to know the melody and chords.
A lot of sheet music arrangements put chord symbols above the musical staff making it easy to know the chords. (Sometimes those symbols are needlessly complex, technically trying to account for non-chord tones in the melody, but let's skip that detail for now.) If chord symbols are not given, you should analyze the music to figure out the chord changes.
When the song is reduced to a melody with chord symbols, you essential have a lead sheet. From the lead sheet you then play the song, not literally as notated, because the lead sheet doesn't provide full notation, but according to different styles of accompaniment and harmonization. A typical approach is to play the melody literally, or maybe with some embellishments or a bit of doubled notes, in the right hand, while the left hand accompanies the melody playing rhythmic patterns based on the chord changes.
The simplest accompaniment is just "block" chords in the left hand, in close position, on beat one of each bar. Other accompaniments might be waltz style with bass notes in the low range on beat one, chords in the mid range on beats after one. You could also arpeggiate the chords in the left hand. If you arpeggiate with open voiced chords in the piano low to mid range, it creates a very resonant sound.
There are lots of ways to make your own song accompaniment, harmonization, and dressing up the melody. Try looking up playing from a lead sheet.
You build relationships with pieces. Just like a woman is not going to appreciate you taking her out just to ignore her. You spend a lot of time getting pieces to a concert level. If practice is boring for you then this may not be the hobby for you. It is how musicians pay there dues and learn there artistic skill. There is no escaping practice.
I see it as when you write or read or even speak a word so many times it looses meaning, try rearranging or adding a few extra notes to make it sound cool, like a few grace notes here and there or try a different chord in the left hand, play around with it. One of the points in jazz I have learned is that it doesn't have to look exactly like the sheet music. Or you could try finding a new variation or arrangement of the same piece, I also like trying to mix songs sometimes (but I'm not very good at it so I listen to it instead)
It sounds like spiral review, or repertoire maintenance, would fit you to a T. When it's gotten boring, set it aside for a while, and then spend a few days with it again from time to time.
In this way you would spend a portion of your practice time most days, reviewing repertoire.
