What did he mean that by 1 day with someone new made him realise how bad things were between him and I?

He is a narcissist and love bombs like crazy

But he cheated and said I won’t come back because within “the very little time” I’ve had with this girl has made me realise how bad things are with you

He met her Saturday and told me this Monday morning …

What does he mean by that? Does he mean sex or?
What did he mean that by 1 day with someone new made him realise how bad things were between him and I?
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