Sis ANKYNEMD I really can't say I am in a position to advise. I just share my own experiences as well as those of others close to me, at baka makatulong naman sa ibang going through trials. Been there, suffered too... we are all sisters, we should have empathy for each other.

Ok, Sis the bad news is an office romance is the hardest to end, IMO, coz there is legit cover to make it go on and on.
The good news -- it is the easiest to buko, coz they get careless -- people around them see and talk. Or, 1 fine day, someone due to grr or conscience prick -- will share the info with you, anonymously.
Or mabuko mo sa FB ng kung sino. It's a small world, MMla.
IMO, your husband and si Kiri (excuse my preferred label) went out, just the 2 of them.
The versions do not match.
1st he says na katuwaan lang yung texting.
Hello, kung anonymous online nakipag flirt ka, pwes puedeng katuwaan lang, na practice mo lang yung carabao French mo. Pero yang magkakilala at nagkikita sa office, puede bang katuwaan lang? There is a named face, attached to a live body, magkakilala kayo eh... Masakit man isipin eh -- nakatuwaan nga nya kasi baka nga patuka yung Kiri.
And he flat out denies they went out, even as part of a group.
Si Kiri naman, she told you they did go out, but as part of a group.
Well, criminals pag hindi makalusot eh di ba admit to the lesser crime. So, IMO -- lumabas sila na 2 lang sila.
Did they go far enough for Kiri to share the insides of a motel with your hubby?
I can't say. Ikaw mag assess -- sa degree of harutan nila sa texting nila.
Ma sense mo yan, how far they went... but do NOT be paranoid ha. Hindi na productive yan by now.
BUT if I were in your shoes, hindi ko na isipin pa yan, que nangyari man or hindi.
Kasi ---- Where you are now, Sis, is the difficult place called The Undetermined or The We Will See place. Hindi talaga ma-say yet if your husband will stay true or ayon, maghanap uli ng lalandiin.
What you can do is hope and pray he will continue henceforth to be a Good Boy.
AND -- you both talked about it and both decided that was The End of that, and move on kayo.
AND -- of course, you will give him another chance to prove he is trustworthy. Mrs ka eh, hindi ka 1 night stand pokz, so talagang you have the power to give him 1 or more chances.
SO -- go with the flow ka lang. Hwag na hwag ka look for mamaws or bring yourself back to The Past with Kiri there -- that is not good for your physical and mental health, Sis.
At hwag na hwag ka mag nag. Hindi ka ang contravida dito, so do not nag.
Ano ba talaga realistically okay gawin mo NOW?
1st nga, TRUST him again. Relax... OO mahirap, I know. But you just have to do it.
2nd, Live in the NOW.
3rd, Always be your BEST. Sis -- que with or without a man, you owe it to yourself (we all do, owe it to ourselves) to be your Best as much as you can 24/7.
Do that: Trust; live in the Now; be your Best -- and let the chips fall where they may.
If Sh1t happens, it will happen nga, even to The Undeserving. Whatever will be, will be.
But see, what will comfort you should Disaster strike is that --
Hindi ka caught flatfooted. Being at your Best is sure to land you somewhere you can take care of yourself. You will not find yourself Helpless.
You will have no regrets kasi at your Best, you surely gave it your All. If he still turned traitor on you, pwes he is dumb to not know or appreciate your worth. Ganon lang yon, Sis.
So be optimistic.
Pray that you get the Love you deserve.