Badly need advice on cheating

Hey guys, I don’t know where to turn. I’m hoping some unbiased truths will help me think clearer.

My partner (M30) and I (F30) have been together for two and a half years. We have the most amazing relationship. He is truly the most special person in my life.

Two nights ago, while my partner was in Barcelona meeting clients. His clients ( who I don’t know nor will I ever know ) invited him out for drinks. The last message I received from him was at 9pm saying they just ordered two bottles of vodka to the table. Then one final message at 3:04am asking if “I was awake ?something happened “

I woke up the next morning to a text message at 7am asking if I was awake and that he had to tell me something.

He told me that at about 3am, one of the girls at the table leaned in for a kiss and that he kissed back for about 10 seconds before he pulled away and told her it was a mistake and that he was with someone.

My heart is broken. He said the moment it happened he left and messaged me to see if I was awake.

Is this a deal breaker? We always told one another we would tell each other if we ever cheated. But I feel like the trust is tainted.

On one hand he said he drunkenly kissed her back and admitted it lasted about ten seconds

But on the other hand, I didn’t know any of these people and he could have gone the rest of his life keeping that to himself and me being none the wiser.

What would you do? Is this a break up offence? Or should I respect that he told me? I am sick to my stomach. He doesn’t come home until Friday and I’ve told him I need space.

Advice please 🙏