I had the same issue really, I used to game purely on consoles and when I started PC gaming as well I was pretty shit at using the mouse and keyboard, just keep practicing and you will get better. Maybe try playing games with less frustrating mouse and keyboard controls than GTA SA? Also a lot of PC games offer support for controllers, I'm not sure if GTA SA does but it might be worth doing, some games just play better on a controller. My advice is play a lot of Team Fortress 2, it should sharpen your keyboard and mouse skills and you will enjoy it a lot too.