Well, the main mistake is investing into the swiss, but that is irrelevant.
Diplomacy is very luck based, you get 5% per chit invested, to get a boost toward your side, and each success eats a chit, so you need to reinvest or the chance reduced. The boost is 5%-8% in most case, with small chance to get 15%-20%. But, the other side can also invest, and the two side cancel out each other, so you invest 8 chits for 40% per turn, they invest 5 chits for 25%, makes it 15% for you, and huge amount of mpp and lots of chits doing nothing. The AI not entirely stupid, so they will invest into the nations that slipping away, so unless you can outbid them in mpps and chits, you are in a deadlock.