Part of the problem is the basic addiction to it, in a sense you're an opiate addict. People who binge/purge bullimia have a similar cycle. Depressed, flat, frustrated, lonely, isolated. The term "flattened affect" describes it well (affect means emotion). It's not that things are getting you down, things could be absolutely fine but you feel no happiness, that's what the term depression really means, it's not sadness it's a depression.
When you cut yourself or purge you basically give yourself an andrenaline boost, endorphins also kick in. Endorphins are the bodies real version of opiates, morphine, heroin, methodine nicotine, caffeine, amphetamine... do you see a pattern? The endorphin receptors in the brain help ease pain and lift your mood. Actually it's not so much a "make me happy" which is usually serotonin, but more a "take the pain away" relief. Which is exactly what you described it as.
So the obvious answer is exercise. Intense exercise. After 5 minutes the endorphins kick in and that should fix it. It's also why people who do regular exercise don't suffer depression as much. Most people can do some exercise and that's fine, others go overboard and are gym-junkies (drug reference right there).
But here's what you have to consider. The symptoms of Depression are the result of a chemical reactions in the brain. But is the chemical change of depression caused by your body regardless of what you think and do? Or does your thinking and behaviour create a chemical change? There's lots of research on both sides and huge debates about it. No one can see for certain.
Personally I take the Dr Phil approach. In all behaviours, even destructive ones, you get something out of it. People often say "I want to change" but then feel they can't and then don't change, and part of that is refusing to admit the positive aspects they get out of something (relief/release in your case). So often people won't change until the negatives aspects completely overwhelm any positive. Hitting rock bottom. How bad would it have to be before you think you'd stop? If you go there now mentally it might help. This is not a scare tactic, just a cognitive (thought) exercise. Imagine the place you would be when releasing you're hitting rock bottom.
Hopefully this should feel strangely carthatic? You should feel less pressure on yourself to stop cutting and think more about what you want. There are other ways to get it and you know that, and you should know you have that choice. You choose, not anyone else. And that's the difference, it's your choice or nothing. I have other helpful tips if this has been helpful at all. Hope it goes ok for you.
Good luck.
