Unfortunately all you can do is give it time. If the relationship can move past this, it will. And if he means as much to you as it seems he does, then I know you'll give him all the time he needs.

But what he doesn't get to do is hold this against you forever. He doesn't get to play this card in every argument just to get his way. So be mindful and don't be afraid to stand up for yourself if you think he's exploiting the situation.

Look...It was a mistake. You obviously completely understand the line that you crossed, so that's a good start. Defending yourself by claiming that it was nothing cus it was just another girl is the mistake so many women make when finding themselves in the very shoes you're standing in. And if you end up getting impatient with how long its taking him to get over this..and you adopt that excuse...you can kiss this relationship goodbye. Because no matter how long it lasts after that door is opened, it will be permanently damaged none the less...and it will erode away a little bit every day until one day one or both of you bails.

Gender is irrelevant in the eyes of monogamy. Now some people are ok with the gender boundary being crossed, but one should always enter into a monogamous relationship assuming its no bueno until you learn otherwise. You fucked that up. Is what it is.

Own it, learn from it, grow as a couple because of it. Those are your priorities right now. Stay remorseful of your actions, but positive in the relationships ability to move past this. Because that faith in each other is what makes a good relationship a great relationship. And should you weather this storm, you'll be stronger as a couple because of it.

And just on a personal note, if I were in your SO's shoes right now, you're acceptance of your fuckup and the genuine remorse you seem to be showing would go a long way in helping me to move past this.