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Frankly you cannot undo the kiss now, it is done and savoured in its own way. your1 girlfriend is upset, and she has every right to be, obvious these thoughts, which are important to know will float through her head, "whether you are trustworthy and deserving of her love".
Now, when all is done, and it is irreversible, time to seek forgiveness, and may it be a sincere attempt.
Tell her that you were CARRIED away for that moment, and you do know that, it is absolutely WRONG on your part, and you FEEL ashamed of your act. But all said and done, she is very valuable to you, and you know, that you are not deserving of her, yet, you cannot be without her at any cost, and she means the world to you. You are ready to take up any penance, she wishes to give.
From hereon, it is for your girlfriend to decide. Lets hope the best outcome for you and her. (i hope x)
At this time, a question to ask yourself, as to what you would do, if she were to come to you, for doing a similar act, "Will you be open to an apology ? and then just forget all about it". This is important to ponder on, how it feels to be in the other's shoes, and what to expect from this episode.
