HALO 3 Solo game problems

Hey i have finished Halo 3 twice now and each time it just comes back to the main menu and says resume solo career or whatever what the heck? I want to play it on other difficulties but when i select resume it takes me back to a checkpoint near the end of the last level

Go to the campaign lobby, go to the difficulty setting, change it, then change the level to wherever you wanna go, and there you go

But this isnt normal is it it shouldn't be saying resume solo game or whatever when i've finished it....

Ive beaten it as well and never got the achivements for beating the game or an achievement for one of the earlier levels.I dont think i got all the armour i was supposed to either.I usually dont care to much about achievements but when you beat a game without cheating it kinda annoys me.

hey i jus got halo 3 and when i choose solo game it always says game failed to start and i cant play