Request couples therapy, if that is still an option. Depending on how he found out and the level of flirting, it may be your best bet. If you were doing some deep-level flirting like sending nudes or personal information...sucks to be you, but play with fire and you get burned.

if you really want to save your marriage than try getting couples therapy like mentioned, BUT lets be perfectly clear here, this is completly on you, if your husband ditches your ass you have literally zero right to complain. and before you go "but it was just a little bit of flirting online, nothing serious...." , tell you what, my ex-gf of 11 years left me not even a month ago for some dude she "flirted with online" , you know how that felt to me ? how it felt to me getting to hear her apologize to me by saying that it "just happened" ? cause that feels like shit, and no matter how often you may try to tell yourself "it was just a little bit of fun" , a little bit of fun is exactly what leads to situations such as this. you were setting up your husband to be the fall guy. maybe it would have remained as "a little bit of fun" with this guy, but who's to say you wouldn't have flirted with another dude a month down the line and that one would have went further ? considering your husband is right at "divorcing you" i'm pretty sure "flirted with" means a lot more than "spent half an hour being silly with each other one time". Seriously, i get the excitement, the fun and all that shit. but even at absolute best doing something like this shows tremendous disrespect for your husband, so no, not **** your life, **** his life for having wasted 13 years on someone who is unable to give him a really basic amount of respect.

I think he might be in the right depending on what the flirting looked like and how long it had been going on for.