aaorn,
A very intriguing question indeed. I wish I had a clear explanation. I myself am confused about my own past. And I think for me that complicated my issue even more.
But, firstly, I agree with what rashkebehag stated that the problem started before Torah was strong in my life. It was not until I was 16 that I became serious in my learning. And it was at that point that I was set on removing myself from porn. But, this porn habit has already been with me for a few years beforehand and I could not get rid of it. I tried and cried. But to no avail. My shaarei teshuvah is still crumpled in tears from those early days of frantic help.
There is more. As much as I try to devote most of my time to learning, not all my time is in Torah. From the time I was young, most of my triggers were in non-learning settings. I would get annoyed with myself for not learning at that time as I knew I would have no desires during learning. I have had may instances where after a wonderful seder of learning I would come home, and realize that I am home alone and out of no where I would get this immense urge, and lose my self completely. I would be so annoyed later because I really wanted to be learning. But I could not control myself. I did not ask for these temptations to come.
I did have a short lived era of my life where I was working full time in NYC. Those were the worst years of my life. My addiction plummeted. I could not even go into details. When I went back to Kollel, where I have been ever since, the problem began to simmer down. In the past 7 years I went through major ups and downs.
So in essence what this boiled down to was a war. I wanted good and bad. I desired Torah and filth at the same time. Which will prevail? So it was not issue of porn replacing torah that made me feel better; it was two opposing forces that attracted me and I did not have the strength to stand up for truth.
But very rarely, did a nisayon hit while I was learning. I loved learning. It was when I removed myself from Torah, even momentarily, that I was under attack. A day that I learned less, I was more prone to porn. Till today, if there is a day that I cannot learn as much, I am frightened. My whole being is much less spiritual. If there is a day that I know I will not be able to learn so much, I put most of the time into mussar to give me strength. Mussar always gave me extreme power. and after each major fall, I would pick myself up with a new sefer and become invigorated. Most of my mussar learning was learned as a result of my porn addiction. But, again, the second I removed myself, the foreign power in my brain would visit and I was no match for him.
I think I am rambling a bit as I explained from the beginning that this bewilders me more than it bewilders you. Sorry if I am not clear about this situation. I will try to give this issue more thought and explain it in better terms. If you are interested, I wrote a lengthy post a while back tracing the situations that triggered the lust in my life.
You can read it here.