Undertaking a genuine Teshuva starts by understanding the mekoros and ikarim of the issurim of these issues. This is not only MY opinion, but many rishonim and baalei hamusar hold like this. You can get rid of an issur and a bad habit by shaping your mind to understand and be aware at all times, of the harm caused by our deeds.
You are talking about what the sforim and ba'alei mussar say about teshuva. That means you know this already. So then why is the struggle so hard if you know it already? After all, Hashem doesn't want this for you. So don't do it. You are a precious child of His - and His personal project. Right? Each of us is.So why do it?
I'm not talking about what's right - I'm asking you about what's with you. (Not what's with the Torah). You is what matters, because we already know that the Torah doesn't hold of looking at porn or masturbating. Get me?
Let's not play games, neither of us.
Teshuvah is b'ikar about what you have done. What does Teshuva have to do with not doing it any more? I know, I know... it's supposed to affect the future... So let it! What's your question?
How about not doing it any more for two months. Then talk about Teshuva.
Not so simple?
I'm not talking about "addiction" here - just about you.
What is the difference in the exact "nature of the prohibition" as far as stopping and staying stopped is concerned, if you agree that it is not what your very best Friend ever (Hashem) wants for you?
I wish I had someone ask me this very question 20 years ago. Oh, how I wish it.
Hatzlocha sweet yid!