Well, there's the problem: you replaced your old monitor with one that requires a lot more horsepower to run (1440p has a lot more pixels in play than 1080p), but you didn't change the GPU. Kind of like taking the engine out of a Chevy Aveo, installing it in the much larger Impala, & then being surprised when it struggles to get above 55 MPH on the freeway.
http://www.techspot.com/review/712-arma-3-benchmarks/page3.html: GTX 770 hits 62 FPS @ 1680x1050 (equivalent to your old monitor), & 51 FPS @ 1920x1200 (near top for your new monitor) at Very High Quality.
http://www.techspot.com/review/712-arma-3-benchmarks/page4.html: GTX 770 drops to 48 & 40 FPS at Ultra Quality.
Essentially, it looks like for this game you either need to add a 2nd GTX 770 in SLI (103/89 FPS @ Very High quality, 80/71 FPS @ Ultra Quality), or replace the 770 with something a lot more powerful (GTX 970, R9 390, etc.).