I know I've cheated but what do i do?

I've been in a relationship for the last two years. I adore this person they are honestly the best to ever happen to me. Last month i went out with friends and then went back to a house party. I was talking to this person who also knows my girlfriend. she was super drunk and i was fairly bad too. Long story short we kissed for approximately 5 seconds. I stopped it and then we both stopped it it was so horrible and so stupid. i know i cheated i know it was stupid and honestly i have no idea why i did it at all. i am riddled with guilt but selfishly i dont want to tell my girlfriend about it. I hate what ive done and i know thats no excuse and she deserves to know but i really do love her and i dont know what to do? am i being super selfish about the whole situation? or am i blowing it out of proportion for something that could really upset her and meant nothing? I'm not worried about her finding out i am just guilty that it happened at all because i wouldnt consider myself a cheater or capable of hurting her. I know it was only a kiss but why the eff did i do it was so stupid and i dont know if ill ever shake the guilt.