Sofie, I used to cut all time - limited to my arms only one - but I know how you are feeling. By cutting, the physical pain helps release the emotion pain you are feeling and while it good when you feel that way after you cut, I do wish you would not go through with it. Cutting, however shallow you mean to do it, is always a dangerous thing, because you can easily mis-judge the depth of when you cut and can end up cutting deeper than you had intially planned and that can severely damage your muscles, your veins under the skin, the endings etc and lead to serious health related problems in life, such as daily pain for the rest of the rest of your life if you damage any nerves and can even lead to blood poisoning, which can cause you major trouble in your body and require frequent hospital visits or, at the least, visits to your G.P.
I cannot tell you not to cut, because that is a personal choice that only you can make for yourself, although I really would hope you reconsider and do something else, like just lie down on your bed or lounge and just cry until you have no more trars left in you, but I can offer you some advice on what could potentially go wrong if you are not careful.