What are you doing wrong that you still struggle with something after years of doing it?
Who mentioned struggling? i didn't.
I don't struggle walking because i practice it every day.
If i keep doing regular exercise i can maintain a decent level of fitness.
If i stop exercising for a few months, my fitness level will go down.
Similarly, if i play piano regularly i will keep a level of technique.
If i stop playing for a few months, my level of technique will go down.
And if i play a piece regularly, i can keep playing it at a certain level,
but if i don't play it for months, i'll play it worse when i come back to it.
I don't beleive anyone can learn a piece to a high level and then return to it after 5 years of not playing it and expect to play it at the same level immediately.