I am sorry this is happening to you and I am really sorry that your DH once he knows of your discomfort and distress is not prepared to do anything at all to alleviate that.
This tells me a lot tho. If he sees it as purely work place bantz platonic he would have zero concern with a totally reasonable request for his focus on you outside of office hours and fade / not reply to her.
Him not being able to give you this tiny compromise tells me where his head is at whether work colleague is in same place or not. It tells me he is either shagging her, fantasising about shagging her or planning to shag her.
His mid life crisis (is she younger than him?) is very selfish and you are not his priority.
The four ‘A’s:
Awareness - you are there already
Anger - it’s valid and an important signal so don’t repress your feelings - it’s also an energy and a clarity.
Acceptance - when you stop begging, ruminating, trying to change someone / something that’s not within your control
Agency - this is your power. To do something whether that’s investigating your options, whether that’s emotionally detaching from him in your head - ‘drop the rope’ whether that’s seeking professional emotional support, whether it’s initiating the end of your marriage. But agency is not more words to him - he doesn’t hear you, you are not his priority. Don’t discuss it any further just have an alternative plan that you are researching.
Never feel you are being unreasonable or irrational. He’s not respecting you or being kind to you - all you get are the moody sullen teenager vibes at home when he can’t be with her - and underneath he will be so contemptuous and resentful of you.