I understand a little of what you're saying I find emotions can hit me like a train sometimes, often when I've been avoiding feeling them. One thing that sometimes helps me is to draw a picture of myself and then draw in red whatever I want to do to myself and then tear it into small pieces and throw them away or otherwise destroy them. Another tactic is the 15 minute game. (Less time if that's all you can imagine but 15mins works well). What you do is remind yourself that you have a choice. YOU HAVE THE POWER, YOU'RE IN CONTROL, NOBODY CAN STOP YOU. When the urge to self injure comes on you, check the time and tell yourself you have felt the urge. If you choose not to SI for the next 15 minutes do something else keep occupied. If after 15 minutes you still feel like SI it's your choice, you have the power, you're in control, choose to wait, choose to hurt....
If you get through the urge and have not hurt yourself, maybe ring a friend to tell them, or email Wedge@lifesigns.org.uk (or not, the choice is always yours). Check out the link
http://www.lifesigns.org.uk/help/ Good luck as you try to ride the wave.