Raul Jimenez had Fulham ahead early on before Villa struck through Ollie Watkins, John McGinn and Emi Buendia to win 3-1 on Sunday.
Sessegnon later said: "We started the game very well then conceded quite a bizarre first goal but going into half-time at 1-1, it's still a decent result but we kind of just collapsed, to be honest.
"We conceded one straight after half-time which can happen but to concede the next one straightaway was disappointing. The game was pretty much done after that.
"There is no real explanations, it's just poor defending from us. Quite unusual from us but we have to look back at the game and see where we went wrong.
"It was probably just lack of concentration, it was very bizarre. We have to pick ourselves up and go again.
"Hopefully on Friday against Bournemouth we can put it right.