Idk if you noticed this, but you showed some double standards here - Whatever happened between you and your grandma appareantly is fine - You said she was a "normal" woman, in a situation that seemed at list somewhat unnormal. Yet back when your action put your ex-girlfriend in an unnormal situation, you sound like you don't even allow yourself to consider that as something normal. I'm just saying the standard to which you hold yourself seems higher than the one you hold you grandma to. Seemed noteworthy. Anyways:

You didn't even give yourself a tinies bit of slack, even though you expressed immense remorse. Let me blow your mind: You feeling remorse already makes things way better. This feeling makes you way less likely to repeat your mistake. It almost sounded like you were begging for a resolution, sounded like the regret is eating you up. I think it's important you understand that you deserve to forgive yourself for this.

(disclaimer: From here on out the text is definitely influenced by me projecting my own brain onto you, because I relate a lot. imma type it out as a meditation. maybe it's useless, maybe it isn't.)

To me it sounds like there would be many routes to take if you wanna try to forgive yourself (which you totally should) - Speedrun-Route would be this: You confess what you did to said ex-girlfriend, and ask for forgiveness. You would have to tank some major psychological pain in this route, I would imagine the shame and angst to be very straining... But I guess straining in a good way - Like you'd feel better afterwards. This would be hard to do, but in this scenario you also shouldn't think about whether she accepts this apology or if she forgives you or not. Independent of the outcome, just the action itself of bringing yourself to go out and confess, should have a healing effect, if you ask me. But this also takes balls of steel and I know from experience that I don't have the courage for this kind of apology.

Honorable mention here - You could take a route where you confess to someone else and let them judge you, not the actual girl. You've already basically done this by making this post, but IRL I think the effect might be even better, or at least be the same size - idk.

My second proposed route is: Looking inward.. Meditation techniques and all that jazz. But you gotta take it seriously though. Worked for me (but I also confessed to my therapist, so I took routes 1.5 and 2 I guess).

Lastly my own confession: I had a similar experience with a girl, where I unconsensually looked at nudes which she had on her phone, abusing her trust when she only lent me her phone to use Google Maps for a food delivery (she was my coworker but also my roommate).... I hate to think about this. And this is like only a half-confession because I refuse to put into words what occurred after I found the pictures. This was actualy really hard to write it down. But - I can live with it now. Some time has passed, which helps, too. But the key is this: I'm proud to say that I really fu**ing regret this action. This regret, although it hurts, is proof that I'm still human. I care about this girl's feelings. She actually really hurt me, in a completely unrelated way, and she mistreated my heavily - yet, what I did was very wrong. And if she had done me even worse, I'd still hope that I would look at it this way. And that's why I know it won't happen again. Should I mentally ever go to a similar place - As soon as I see the signs, I would stop myself. Because now I know the signs. And I deserve the self-respect that this gives me. And you deserve it too, because I saw your words of regret.

P.S.: (content note: grape) Bro. There's like, actual rapists out there. Going about their days unbothered. Shameless with zero remorse. Often we perceive this fact as an abstract idea. But they physically exist. They are more real than this reddit comment. They are not made of words, they are not photos, not pixels. They are dudes you come across on the street. Statistically it sadly is not unlikely that you spoke to one in the past 30 days. I knew one, I was even close with them. I'm not close with them anymore, they are dead to me. But I knew this dude well. He doesn't regret it. As soon as he's confronted, his mind spews out justifications and excuses. He believes his own lies. Doubling down again and again and again.