I want to join

. A military or naval command would be great.
My current advice is strategic: ignore the fact that france holds onto our lands in Brittany, France is simply too hard to hurt, until we've got a massive colonial Empire....
Build up a good fleet (you raised war taxes?), mass it, and use it to support landings on Norway. Destroy the Norwegians.
At the same time seize all of France's colonies, normally poorely defended, and of course hard to support from the mainland.
Once Norway has been completly taken out, begin to work on the Danes. Both these nations are quite weak.
Now at this point, you're probably able to demand something from France (alliance leader). Possibly removing their cores from your lands.
or you could get a seperate peace with norway/Denmark if you want to expand up there.
As an aside: I currently have a game where I am scotland and I own 3 provinces on frances west coast, and I don't bother defending them, i just eat up all the rest of Frances colonial holdings/her allies. [heck, by mid game I actually had large enough colonial holdings that when I got in a war with england and their alliance [Portugal, aragon] AND france and their alliance [Castille, Norway] at the same time. I abandoned Scotland, painfull I know, but managed to secure a white peace by having a merry old time seizing colonies and so on. in short, if we cannot defend it, don't. Hit something else, losing control of Brittany is tempoaray, and we can gain warscore elsewhere.