Dear OP:
The problem is that mortal sin temporarily blinds us to our depravity and our failings. (That’s why I highlighted the above portion of your words.) It desensitizes us to our sin and leaves us more in the grip of the Devil, who is the King of rationalization. ('Can’t be wrong because it feels good, it’s “natural,” “it’s difficult to resist such a bodily temptation,” “It’s not hurting anyone,” etc.) And the more one becomes habituated to any sin, it becomes increasingly easy to rationalize it.
Masturbation isolates us from the natural order of sexuality, which is paired, not solo. It also unnaturaly focuses us our ourselves, turning the sexual act inward rather than outward. In doing this, it perverts sex and distorts its purpose. It also makes two idols (which is against the First Commandment, of course): we idolize ourselves by it, and we idolize pleasure for its own sake – as opposed to pleasure as a gift & as a product of holy union with the human object of our love. In pleasuring ourselves, we seize this gift out of impatience and false entitlement, therefore devaluing the gift itself.
Sin wounds us, leaving scars. Our relationship with God and His Son are healed through the sacrament of penance, but we may still battle with our previous inclinations, if we have become habituated. We also, though made whole by the sacrament, may have feelings of remorse, shame, regret – as we can with any sin of any category, committed recently or long ago.
I wish you would read Jason Evert: that is the advice I leave with you. Both he and his wife Cristalina (sp?) have “pasts.” He and she have mentioned this on EWTN several times. He was addicted to pornography, she to premarital sex and lying about it to her parents. Both of them still carry the after-effects of that, to some extent, and are up-front about that, for which I respect them. They don’t try to spin themselves as perfect or as without temptation or a fallen nature. Therefore, their attempt to live purely since their conversion is all the more credible. Jason’s books are advertised here on the CAF home web page. I think you would relate well to them. He is not a condemning person; he is an honest person who tells it like it is, unflinchingly. You might start with Jason’s book, “Theology of His Body/Theology of Her Body.”
We are all wounded. Some people are less aware of it and less honest about it. I know for a fact – I have experienced this – that there have been ingrained tendencies in my past that now make it more difficult for me to extricate myself from these bad habits, some of which are grievously sinful but which I used to be unaware I was engaging in so often out of force of habit. They key to losing any habit is first, to gain awareness of your actions and motivations, and second, to pray about it. The more tempting the habit, and/or the more ingrained the habit (which is another reason to conquer it a.s.a.p), the more personal prayer we need to battle it. Thirdly, frequent confession.
I don’t want to go into more specifics at this time about remedies, because those have been discussed often on other threads to which I have already contributed, and I’m sure you can find such discussions here. I’m merely trying to answer the specific questions you asked on this thread.