Time to let go of this porn thing.

At the time you needed it. She had no desire whatsoever in meeting your needs. As long as you see it as bad or a mistake, she will continue to use it to control you, either thru shaming you about it, or that you "cheated" on her.
Next time she brings it up, say darn right, and I enjoyed it. Might sound cruel, but she will eventually come to the understanding that she can no longer use it to control you.
While you hold the "shame" within yourself, she can use it against you. Time to let it go. Forgive yourself.
In actuality, there is nothing really to forgive, except that she has you convinced you commited the gravest sin known to mankind.