I've experienced the same result (RE: the Brit Corps) when I forgot to send out the navy.
I agree, the AI is very good at counter attacks (who are those guys, the Germans in exile?).
The furthest I've gotten is mid 1915, & I've captured Paris, but the French aren't out yet.
Keep the right strong (remember, that's the Schlieffen plan!) & keep going hard, manage your HQ & refit pts carefully, & use plenty af arty.
The trenches DO make quite a difference for the defender.
Also, put pressure on the French in the southern sector when they start to shift troops north.
They'll still shift the troops, but you may get some ripe plums dropping into your lap, such as Verdun, if they weaken it too much (which happened in my game).
I found out you can't capture India with the Turks, but you can go after Egypt fairly easily, but the Caucasus may be a more effective route to take.
Spend Diplomatic points an Italy! Once they enter for the Entente, the Austrians have a very hard day (every day!).
Also, remember that unactivated cavalry CAN advance into unoccupied enemy hexes when stacked with an infantry corps. The Russians play this little game all the time (very annoying)!