If he was honest, he deceived himself a whole lot more than he deceived other people.
When he finally married Lily, he deceived himself into thinking that they would spend forever together. He tricked himself into believing that he would live a long and happy life with the girl he loved, and that they'd share a bright, exciting future. He was wrong.
When they had Harry, he deceived himself into thinking that he'd watch his son grow up. He tricked himself into believing that he be there forever for his little boy, to watch over him, and guide him, and keep him safe from harm. He was wrong.
When they went into hiding, he deceived himself into thinking that maybe they could outsmart Voldemort. He tricked himself itno thinking that everyone shared his storng morals and fierce loyalty, and that everything would work out fine.
Unfortunately - as Peter proved to him by making the worst betrayal a friend could make - he was wrong.