I think a lot of it probably had to do with Harry. He was absolutely horrible to him, and probably visualized things he wanted to do to him or his dad to do to Harry. He may have been seeing this done to himself even!

Haha--maybe his worst memories are of being put on the atrocious diet his mother put him on.

On a more serious note, I think Dudley's worst memories weren't memories at all, but rather worst ideas--like Harry being more important to the family than he is, or his parents dying, or something to that effect. That was a wonderful question. It has me stumped.

I think it was sth serious, we dont know his school life. Getting in a huge trouble in school? Or maybe being the fun with his weight until he did boxing?

Hard question, but I keep on thinking about the time Harry trapped Dudley in the zoo with the snake.

who says it's just one memory? maybe when he's around dementors he remembers EVERY event that he was in contact with magic like hagrid, fred and george, and harry with the python at the zoo.

It is possible that his worst moment was the Dementors themselves because for Dudley it was even more terrifing because he could not see them and could not fight back.

I think Dudley saw himself for what he really was and that's what brought about the change, but I'm not too sure.

I think that Dudley saw himself, as everyone else saw him.

Oops, didn't notice the person who put their view down before me...

probably looked in the mirror! If I was him I'd be scared...

So true. He saw himself for what he really was and knew he had to change.

It could have been the memory of getting locked in the snake house in the first one. Where the glass disappeared and he couldn't get out. He might have been reliving the experience of panic; feeling as though he is trapped and can't get out?

He probably had some worst memories. Like the way Harry can make things happen without them meaning to; so Harry might have made something bad happen to Dudley. Or maybe he did something extremely bad to the dursleys when he was younger which made them turn horrible towards him.

I think that when the dementors attacked, Dudley was reminded of all the horrible things he had ever done to Harry and maybe he was finally feeling a bit guilty about it. In DH, Dudley is the only one to show some sort of goodbye to him.

I agree with everyone saying that he probably saw the truth about who he really was: a spoiled, bullying brat.

I'd just like to add something that Dumbledore said in HBP about how Harry, at least, avoided the terrible damage that was done to Dudley by the Dursleys. Vernon and Petunia looked confused, but Dudley became embarrassed. I think he understood what Dumbledore meant, based on his memory of the dementor attack; That the Dursley's spoiled Dudley so badly that they turned him into a nasty little brat, and it made him feel guilt.