Just be honest about it. Acknowledge that the kiss happened and that it might have been a bit unexpected. You can say something like 'I'm sorry if that made things weird, but I just had a moment of... I don't know.' This can break the ice a bit.
This is a really tough situation. First, you should be honest with your girlfriend. Admitting your mistake is crucial, although it will be very hard. She will likely be extremely hurt and angry, but hiding it will only make things worse in the long run.
Well, you've made a huge blunder. You need to sit down and have a serious talk with your girlfriend. Explain exactly what happened, why it happened. Maybe it was a moment of weakness or confusion. Then, be prepared for any reaction from her, whether it's anger, sadness or breaking up. And you should also apologize to her best friend for overstepping boundaries.
Well, first of all, don't panic. It depends on how your friend reacted. If they seemed okay with it, you could have an honest conversation about your feelings. Maybe you both have deeper feelings for each other than just friendship. But if your friend was shocked or uncomfortable, give them some space and time to process what happened.
Apologize sincerely. That's the first step. Tell her you know you made a huge mistake and that you're deeply sorry. Then, give her space to process her feelings. Don't pressure her to forgive you right away.
It's also possible that the girl's best friend is an older person, like an aunt or a grandmother. They could have a special bond where the older person gives the girl advice and shares stories from her own life, and in return, the girl brings youth and energy into the older person's life.
You can make it unique by highlighting a rare or difficult situation you both overcame together. So a title like 'Surviving the Storm: My Best Friend and I' would be good. It could be about how you supported each other during a natural disaster or a personal crisis that not many people experience.