I switched from console to PC a few years ago. Its worth it to stick with the Keyboard and Mouse, but if you really find yourself having trouble, getting a gamepad like the Razer Tartarus or Orbweaver can be a good midway point. I used to only play with my Mouse and Tartarus, but eventually I stopped using that and got used to straight up Keyboard and mouse.
I have played minimal games on my PC and this is the first FPS. It is definitely a struggle at first but by the time I ran all three characters through the story it was miles better. I'm not sure I can even go back to a controller now.
Spend alot of time adjusting everything to you. Setup mouse dpi, make macros, and set key commands to your favor. Youll thank me afterwards. Its tedios, but very rewarding, especially if you practice with your setup the way you likee it