If you have pain on the corner of a nail, it most often means that the nail's corner is curved in and pressing into the skin next to the nail. Trimming these corners can be futile for the following reasons. 

1. It is extraordinarily difficult to see clearly down the corner of the nail, so it is very likely you may trim down and leave a piece under the skin that as the nail grows will cut into the skin and cause further pain and sometimes infection.

2. If you try to trim the corner back, little by little you may get to a point where you have cut too far or have cut the skin, which opens up the skin to developing infection.