Self harm no one cares why's it not something people discuss?

Basically A few months a go I cut my arms really badly and some girls at school found out (it wasnt the first time but it was the worst and most noticable), but i didn't know they told anyone. Last night at a sleepover with my mates, i found out they'd told eevryone and my mates had known all along. Why didn't my mates try and help me?! Why's it not something people feel comfortable talking about? and my 'best friend' whos actually a total ***** said 'why would you do that?' but in a really horrid mean voice and no one understands! I havnt done it for a while but knowing something like this is making me want 2 do it again :(