Yes, masturbating is a terrible thing to do, as Chazal tell us. Er and Onan would agree, I believe... And, surely, having sex with myself (that's exactly what masturbation is, of course) is destructive to me on many levels. And yes, we Yidden ought not to do it.
But that has nothing to do with whether it is an addiction, as far as I know. Why does it matter to you? If it's not an addiction, does that make it OK? And if it is, does that make it OK? And if yes to either, who really cares? It is your life that is going by while your brain watches and analyzes it. Is that analysis an addiction, perhaps?
So...
Dang-nab-it! Quit playing around, and discover yourself already! Nobody can answer these questions for you! And if they could, they still could not answer them to your satisfaction! And that's normal!
So, perhaps try going 60 days without it. Not as a challenge, c"v - that's not the point of it, at all! It's not about right and wrong or aveira/not aveira... It's about the facts of what it does for you. Stay away from it for 2 months to know what your life is like without lust and masturbation! When you take away the medication, the illness declares itself!
And if you are not using lust and masturbation - just enjoying it (cuz it feels good, as you say) - then what big deal will the loss of 60 days without the great pleasure cost you in light of such a great experiment?
Stop being a 'poseyach al shnei has'ifim' and go for it, man!
I love you!
Dov