What’s your motivation for starting the endeavors in the first place? If you’re willing to give something up because it’s too easy, you are probably thinking about it incorrectly. Becoming the world’s number one pianist is not an easy task, it would takes years, or a lifetime.

Regardless, if you were on track to become the world’s best pianist and then gave it up because guitar is “harder”, why did you even want to be the world’s best pianist in the first place?

It sounds like you are dabbling in a bunch of things trying to see if somehow the learning of a new skill will give you a sense of enrichment. It will, however, that will fade quickly.

After learning a new skill and becoming “good” at it, there’s a lull phase where you think things are easy. That’s because you don’t know what you don’t know. Mastery of almost anything is hard and takes a long time.

All of this returns to motivation though. If you are looking for a short term sense of accomplishment, then dabble in a bunch of stuff and get “good” at it. If you are looking for a pursuit, find some motivation that will drive you to try and master something.