This is a problem caused by physics. There are many possible tuning methods, and all are compromises of some sort. On a fretless instrument, it's possible to make tiny adjustments so that it plays in tune. On a fretted instrument, or on a piano, that's not possible or practical. Equal temperament tuning was invented to make those instruments play in tune in every key, but it's not perfect. One can tune pianos or guitars so that they play perfectly in tune in one key, but only in that one key, and other keys are even more out of tune. Equal temperament just puts all keys out of tune the least possible amount. If one wants to play in multiple keys then that's the best anyone has come up with. If one wants to play in only one key forever, then it's possible to get the tuning very close in that one key, but the rest will be more out of tune. If you can't stand the slightly out of tune sounds, then the guitar is not the right instrument for you. Nor is the piano. Get a violin or a cello, on which you can, with long practice, produce music that is perfectly in tune. I don't have time to learn to do that.