You have oversimplified the function of dopamine to the point that you have said something misleading. It isn't a drug. You also get a "hit" of dopamine when there is a loud, surprising noise, and any number of similar situations. Including good, natural situations. Including normal sex. This doesn't mean that "your dopamine levels get out of whack".
When you say that 'life just becomes more dull' and 'distracting yourself from negative emotions' it sounds like you are describing dysthymia or depression more than anything specific to porn. The link between monoamines (like dopamine) and depression has been known at least since MAOIs were first prescribed for depression. Of course, if you feel terrible about yourself for looking at porn once a week then that won't help any ongoing problem you have with negative self-image and depression.
Edit: in other words, I suspect (though I can't at all prove) that you have the arrow of causality reversed, and it wasn't porn causing your depression