Excellent article that applies to all instruments, and not only to the piano. (And it’s not too long, either!) Click Here to read it!
Not much needs to be added to the article, but I do want to make the dreaded comment: learning music is not always fun.
FUN in music is being able to perform well on pieces you have learned, especially with others. Sometimes you will be frustrated (particularly if you aren’t practicing well), sometimes it will be tedious or irritating, and that’s completely normal! Take a break and do something else, play something different, and then come back to it in a few minutes. A practice session for a beginner should be about 20 minutes of playing (not setting up) and then take a break and come back to it, unless they are making progress and want to practice longer. Some advocate no more than 45 minutes at a sitting (due to the brain’s processing ability). I encourage two 45-minute sessions for advanced students, rather than one hour-and-a-half session.