I know what you mean, as far as the attachment that can't really go any further, it is a really sucky feeling. I'm sorry it makes you feel the need to cut though, I've done similar in friendships for similar reasons, grieving a love you could never be honest about.
I wish i had any real advice, apart from finding distractions to give yourself the chance to not self injure, like coloring in, or drawing something that makes you happy where you would cut instead, but i do send hugs and support. This is not easy to get through, I know, but you can.