Every time you start any particular campaign it will be a new one unless you load from saved file.
If you have finished a few of the campaign mission so that subsequent missions have been unlocked(campaign is just a linear mission progression) you can start over by selecting the first mission.
If you are into contemporary submarine warfare then DW is your game. And you MUST try any or all of the available supermod. LWAMI or DWX now that the French and Italian ones have been suspended and they are now playing with DWX.
The FFG can be rather daunting to master but knowing this should not make you be afraid of learning the other relatively easier platforms.