This is one of my favorite Jack scenes. He’s trying to explain to Ten what it’s been like for him, to understand why this happened to him, to understand why he was abandoned. All while standing in something that should be killing him, but because he can’t die, he’s the only one who can do it, save these people.
And can you imagine waking up from starving to death? That has to be the worst.
Then, after rattling off a list of the many ways he’s died (and I doubt this is all of them), he looks the Doctor in the eyes and tells him “and all that time you knew.”
And the only answer he gets is “That’s why I left you behind.”
Jack looks away and you can see all those years, all that effort to be the man the Doctor would like him to be, and he thinks maybe it wasn’t enough.
I also think that’s the moment that a lot of his hero worship/being in love with him died. There will always be a little, but when you’ve waited for someone for over 100 years and all they can do is try to run and then tell you they ditched you on purpose, a lot of things gain perspective for you. I think his plans for the future start to change and he realizes how much he overlooked in his life while pining for the Doctor.