Basically you are self inflicting on yourself. The cutting is a way of helping you deal with emotions like stress, anger or sadness. Do you feel a little better or somewhat relived when you've finished cutting?
Anyway, it is something that is frowned upon, which is why as you said some guys stopped liking you.
What you have to be careful of is the dangers, where you cut and how deep, could be the difference between life and death. If you cut too deep, you risk losing a lot of blood. And if you cut a vein accidentally, you may as well say goodbye to your life, if you don't get help asap.
So stop, instead of cutting, you need to find another way of dealing with your emotions, of releasing them. I strongly recommend that you do counciling. It may be a bit tacky when you start, you may find you have trouble talking about your life to a stranger, but it does get easier. By talking you will be able to get those emotions that you've built up inside of you, out.
PE of a friend