I don’t know what to do or think!

So I’m gonna keep it long story short

Me and my boyfriend have been with each other for just over a year and a half, my daughter was playing with his phone and clicked on a chat that appeared.

On this chat he was messaging another girl saying “bad girl,I should spank you” and she replied “please do “

and I have confronted him about it ,apparently he has known this girl his whole life and it was nothing like that and he loves me and he is so sorry that he even sent that he don’t know what he was thinking.

I told him we can try and move past it but I can’t get it out of my head and I keep thinking about it.

He has been an amazing boyfriend up until now when I saw that .

What should I do.