@Simonb I definitely get that and I don't think you are unusual. My father started playing when he was a child (different instruments, none of them piano). Of course he became a professional (solo performer, in an orchestra and later full time teacher) and then when he finally retired, he kept on playing for pleasure. Even when he became too sick and too weak to do anything else, he still managed to pick up his instrument every single day until the day he passed. Some people are simply born for music 
How do you get over a motivational slump?
Try working on something new - and not too difficult that can provide more short-term rewards.
If you can figure out what is causing the lack of motivation, that will be a clue about how to proceed. Are you frustrated with lack of progress? Are you working on too many pieces? Are the pieces too difficult? Are you tired because of other demands, e.g. work? It sounds like a good time to step back a bit from piano, take a break, and assess how you want to move forward. There’s nothing wrong with taking a break from piano.
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pseudonym58 This 
I'll go purchase something very different to my usual pieces at sheetmusicdirect or JW Pepper. There can be lots of arrangements to choose from, starting at very easy to more advanced, and sometimes an official score. I currently have a Hozier piece to turn to when my baroque pieces wear thin. Certainly cheaper than a fresh tuning, never mind a new piano or partner 
For me, when I’m not excited about playing but feel I “need to,” I ultimately don’t rely on motivation and instead treat it as a clinical, straightforward activity similar to brushing my teeth.
It works for me for things like healthy diet, exercise, etc. as well.
You start by taking a break.
If the motivation doesn't come back by itself, you think about what would be the most pleasant thing to play/practise on the piano, and you do that.
If you are still not motivated, time to analyse what it is that made you lose your motivation. Were the pieces you practised not interesting enough, were they too easy, too difficult? Did you press yourself too much? Or isn't it anything piano-related - is your life away from the piano too demanding?
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... feeling like the pianist on the Titanic ...
Finding new & interesting pieces to play including going to a piano store, downloading sheet music online.
Usually, it's a deadline. I HAVE to learn a particular song or songs for a particular gig.
I'm in a place I haven't been before. For decades, I did a lot of church work and could run nearly on "autopilot". I knew pretty much all of the songs that would be thrown at me, probably played them hundreds of times. Would use most of my creative stuff for offertories and postludes. But now, the church work has ended and I've entered a realm where what I play is generally paid work. No, I'm not getting rich. But it's not just for fun anymore. It's a different dynamic than I've lived with previous to the past two years or so. (Church work was strictly volunteer other than the occasional "special occasion" when they'd give me a little pittance. It's not that many years ago that I realized that some church musicians actually get pay for their work. Anyway, left that world behind and have no intention of going back.)
There is a local church planning on a fundraising event. A friend asked me to play some music as a duet. I promised to do up to 5 pieces. Learning them in a month is the motivation.
A lot of people would learn assigned pieces for a teacher. We talk about bringing pieces up to the performance level. The majority are not planning to perform even for relatives & friends. On the other hand, you can always submit a video / audio recording to the online forum. Even a short piece would give you the incentive to work harder to get the best possible sound.
thepianoplayer416 A friend asked me to play some music as a duet. I promised to do up to 5 pieces.
The few times I've been asked to do something like that and presented with the "sheet music", I got to the point where I declined. If I wasn't able to "feel the music" such that it just came forth naturally, it was nigh unto torture for me to learn stuff I usually didn't even like note for note and have it sound good. It's not that I couldn't, but I REALLY did NOT want to.
I'm still shy away from printed music. I'm not exactly sure why. I can read it. Then again, there are some music styles that I can play but loathe to the point I can't even listen.
Guess that's what happens when ya get old and grouchy like me. LOL!!
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Music in church is mainly to support a choir or 1 or more singers. Unless it’s a talent show, the music would be religious with the focus on the singer(s). As a performer you may be allowed to choose your pieces depending on the occasion.
I belong to a music group. I don’t always get my choice of pieces but keep up with practice. Everybody in the group enjoy music & playing together despite not liking some of the pieces.
You come across unfamiliar pieces all the time. Even when learning with a teacher, you don’t choose many of your pieces. If you only play the pieces you like you wouldn’t be doing many of the teacher’s pieces.
Somebody online brought up an interesting point many students quit piano because they’re learning pieces they don’t like or not related to their life experiences. If corse there many different reasons why someone would quit including being forced to learn as a child. Part of the learning process is to try different style & genres of music. You’re not going to like all the pieces.
I don't gig and have no musical commitments. I just play for enjoyment, or, creatively speaking, because I can't help myself. And there are times when I don't feel like playing piano, either because I'm bored with what I sound like, or just feel like "why bother?". I also play guitar, so sometimes I get involved with that at the expense of the piano. Usually, I gravitate back to the piano, maybe because I've had an idea, or heard something that reminds me of something, or something else. But what really makes me want to sit down at the keys is listening to Bill Evans. Even though I don't play like Bill, and never will, sometimes I just feel like I want to make some of those sounds, the feeling that comes from creating a voice, maybe even a single chord. The goal at that point isn't to try and play like Bill, but to try and capture the essence of how Bill expresses a melody or phrase or combinations of notes over a chord. Once that impetus strikes, I'm good for while, until I get bored with my playing again.
PianoMonk , I don't think I'll ever play like Bill Evans but I have come to appreciate some of his style. I do regularly play Waltz for Debbie and have messed with Very Early a bit. Neither are all that difficult though the chord progressions are somewhat unique and don't always... well, ok, rarely go where they might be expected to. But they are both pretty tunes.
Bill Evans left us too soon. He had a troubled life in lots of ways. But he left behind some good music.
Just submitted a piece to the Online Recital. The next closest thing to performing on a public piano is to make a recording of yourself playing and sharing it online. There is a deadline so you push yourself to learn pieces or pick easier pieces that can be learned in a day or 2.
When pressing the 
button, we can do several takes and keep the best performance. On a public piano you only get to play a piece once even if you have wrong notes.
We were "hotdoggin'" a bit in the band last weekend. Somebody requested a song, Mack the Knife. We don't have that one on our list that we've practiced but it's not that hard of a song. So, I started it out and played it in C. Mmmkay. And then the drummer was clownin' around and daring me to bump it up a key. So I did. C#. Played through it just fine. And the drummer dared me to do it again. And since D isn't a favorite key of mine to just jam in, I bumped it from C# to Eb and played it again. Plenty of wrong notes in there if anyone was paying close attention... but we were laughin' and clownin' around and the crowd was eatin' it up.
Wrong notes aren't the end of the world in a live setting like what we were doing. If you're in a recording studio and want every note perfect, then I suppose wrong notes are a much bigger deal.
I remember of a lot of wrong notes when I'd listen to Victor Borge play. I didn't care. The audience didn't care. It wasn't about "correct notes". It was about how he, as an artist and performer, made the audience feel.