I think all of us are familiar with this feeling! Perhaps try this, in regards to playing piano, you could aim to practice every day. Now, when I say that, it doesn't mean an hour or more of intense practice. Instead, just aim to PLAY it every day. It could be as little as 5 minutes, and you could do that every day. That way, you still have that sense of achievement.
This is what I do with guitar, if I'm bored and I don't know what to do with myself, and I don't REALLY want to have guitar practice, I'll often pick it up and play it for perhaps 10, 15 minutes. Afterwards, I feel I have made a bit of progress, and I'm happy that I've done something good and productive!
When you think, 'Oh I'll just play it for a few minutes,' it seems much less intimidating, and you are more likely to do it.
Also, it doesn't really matter if you miss a day or two; I don't play guitar every single day. Going back to it is the main thing!
I hope this little tip has given you some encouragement :)