By the rules, that's it - on a given turn, you either conquer or go into decline, and if you choose to conquer either you just sit there, or you pull all your tokens off the board and start from another edge. Neither option will really get you unstuck, since even if you start over, you won't be able to conquer many territories your first turn, and your opponent will have plenty of time to box you in before you can expand widely.
As far as I can tell, diplomats are just kind of broken in two-player games. If you get them on the first turn, paired with a race with a decent number of tokens, you can conquer through the center of the board, splitting up what's available to the other player, and likely guarantee victory. If they come up later, it's probably okay, since you're unlikely to be able to cut the board in half without touching your opponent's active race.
(I've had one game like this, and in the future I may just remove diplomat in two-player games, or at least veto it if it comes with one of the races capable of splitting the board like that.)