so in 1082 we learn that buggy remembers and holds dear roger's vision/will but expected shanks to follow it, as he thought shanks would succeed while he would not.
did he have any good reason to think he himself would fail? like, any tangible proof of his limited capabilities?
well...no?
because in practice, buggy has a lot going for him. for one, his luck is extraordinary. his charisma is off the charts - he arrives at marineford and whitebeard himself asks him to become his ally! he's not stupid - he can come up with clever plans and even as a child he was observant and smart enough to ponder how teach not sleeping ain't normal. he can strike up a friendship like it's nothing (see: ace). he's decisive and ambitious. he's realistic and not delusional - he understands that some things have gone his way 'accidentally' and is honest enough to say as much.
and for sure he loved roger. and the remaining roger pirates don't have a single bad word to say about buggy but... it seems they never encouraged him in any way? it seems like buggy was legit raised in an environment where it didn't even occur to him he could achieve something on his own merit. the oro jackson adults ain't looking good here.
but i think there's a possible explanation as to why they didn't want to encourage buggy's ambitions or show support for his dreams... buggy is xebec's son and the roger pirates knew this. they raised him as their own but they thought that if they fuelled his ambitions and encouraged him to believe in himself, he might turn out like his father, who was ambitious but in a bad way. so instead they went with the 'be humble and be happy for your friend shanks' success!'. and it seems he was - except that it also meant he'd end up 'delegating' his own dream (which he, of course, did have!) to shanks only to be disappointed by his friend's bum attitude. so he became unhappy and kind of lost 'his place in the world'.
it's quite sad. roger('s crew) kinda fucked up there.