The best option for you right now would be forget about exp gain and concentrate on credit gain. For that, Blitzkrieg is what you should play.
Blitzkrieg is very good for earning credits, but not so good for exp. If you keep playing the other game modes and specially GB, you will get very good exp, but you won't have the credits needed for your next ships.
I know its a hard thing to do now that we have an event, but seeing your problem, it would be the best choice. Your wins will still count in blitz and you would still be able to get the boxes, just not the extra exp.