If she hadn't left you, would you have carried on watching it?
If you think that she assumes the answer is yes, that's why you can't rebuild trust.
So often, people don't stop doing something they know that hurts the other person until it personally affects them in some way and has direct consequences. When they show an interest in rebuilding things, what they actually want is to get the person back and benefit from their company again. There's usually a flurry of regret about what happened, and then they return to their old selves again, once they think everything has simmered down.
Not all women have an issue with porn, and many men watch it, but I still don't think it should be a given that women need to know telepathically that their partner watches porn, especially if it's regular, and especially if it's something like OnlyFnas or salivating at women on Instagram.
If you want to get married one day, I urge you to think about what the phrase 'forsaking all others means'.
Many men don't. For them, monogamy is not sticking their penis in another woman, but interacting with other women sexually or giving them their sexual attention is totally fine.
Just food for thought.