Well, it's rather the AI, which puzzles me. If they can force any (separate) peace upon me, why don't they do so as long as I still have only 1 province from Bretagne? No, Bretagne waits, until I get a second one (I have to lay siege on it since the Bretons allready threaten to re-conquer the first one - leaving me without any occupied province at all, which of course also means game over). And of course I am only at 16% war score or something with Bretagne, when I get the second province, so I can't get any separate peace which would provide me with one of their provinces (they are worth at least 8%).
So there seems to be no way out: As soon as I capture the second Breton province I am doomed. As I am if I lose the first one. Weird situation.
(separate peace-treaties with Burgundy or England do not work - not enough money to buy them)