I also want to address another point. You wrote that you view porn when you will anyway be having relations with your wife that day, and therefore it seems to you to be less severe. Many people don't know this and I am not blaming you, but this is a grave distortion in the proper (and Jewish) outlook to sexual relations with one's spouse. Viewing porn turns all women into objects of lust. This is the very opposite of what relations in a Jewish marriage are meant to be. The "lust" aspect of sexual desire was created by G-d to cause a man to seek out a woman and take her for himself. Once a couple is married, the "lust" aspect of sex should be relegated to the side and should no longer be a dominant aspect of the relationship between the two. They have dedicated themselves to each other, and when they are together, they should be cultivating an emotional bond through the sharing and sensuality of intercourse. Sex is a "sacred moment" of closeness between the two of you. This is also perhaps why Chaza"l prohibited having relations by day. Firstly, because one may see something about his wife's body that may turn him off, and on the other side of the coin, by seeing his wife's body he is fueling his lust instead of focusing on the true sacredness of the moment.
A man who insists to make "lust" the most dominant aspect of his relations, especially by viewing porn before hand, is losing out on the true connection and closeness that martial relations should cultivate, and ultimately his marriage is likely to fall apart. As his wife has children and her body changes, and/or as she gets older, he will loose interest in her and he will never be able to lead a normal marriage life. A relationship built on "lust" quickly fades away. Also, by fueling the lust through his relations and not focusing on building the emotional bond with his wife, he will continue to be pulled after the other women he sees on the street, and will transgress all the time the Issur D'eoraysah of "Lo Sachmod". That is why once a person is married, he must work on relegating "lust" to the side. It is truly a poison, and the nature of lust is that "the more you feed it, the more you need it". One can never get enough of lust. In the end, either we win it over or it wins us over.
To summarize, we can't stress enough that sex between a husband and wife should foster an emotional closeness and bond between the two of you, and should never be viewed as a "lust" based act. Viewing porn before sex, besides the serious prohibitions that one transgresses - as mentioned above, will either destroy one's marriage altogether, or in the very least, will remove any potential for a deep, emotional closeness in his marriage.
I hope that my words and the words of Rabbi Twerski will enter your heart and help you resolve now -- in Chodesh Elul -- never to go back to these things again. If you heed our words, you will be worthy with Hashem's help to bring holy Jewish children into the world and raise a family based on holiness and purity.
Just one last thought. Even with a strong resolve, one should never trust themselves in these areas and should get a strong Internet filter for their computer to help them through moments of weakness. And never stop davening to Hashem to save you and your offspring from what is perhaps this generation's biggest test!
May Hashem be with you.
See next page for an original Rambam speaking on this exact topic.