Take a breath Hereigoagain45 . I mean this kindly because anxiety is horrible especially when it concerns your relationship with your other half.. and can lead to a horrible spiral of suspicion on your part and what you see as secretive behaviour on his part.
Let us look at this situation from a different angle.
You both had a work colleague. You didn't like the tone of the messages especially the xx on the end of them.
This tells me that you may be reading more into this than is actually real.
xx is such a common sign of in the world of messaging these days that it means nothing more than friendship. I am sure , like me you xx in messages to friends and Close colleagues. If I message a work colleague about a work matter then there are no xx.. however I could just as easily send a text or email about an out of work meet up as friends where xx is appropriate. This goes (for me) for both male and female colleagues/friends.
However, you have suffered a previous betrayal and are naturally hyper-vigilant to signs of such an awful scenario playing out again and react based on your experience. The colleague in question is not you and won't be aware of your extreme sensitivity in matters that involve interaction between your DH and this colleague. Neither does your DH . You say yourself that you do not believe he has cheated. Therefore he has no reason to believe speaking to this colleague with her xx is anything to be bothered about. Until you 'kicked off' about it.
So now - he has a choice . Does he dump his friendship because you don't like her speaking to him ? Or does he keep the friendship away from you believing it is the right balance between not distressing you and upsetting you further ?
Unfortunately this is a common mistake because secrecy no matter how kindly meant - often reveals itself such as now - and leads you down that rabbit hole of 'what does he have to hide' ?
However, the other alternative is to stop his friendship e cause it upsets you. Despite there being no reason for this suspicion as you say yourself - you don't believe their is an affair. This is actually quite controlling. Where does it end ? That he may only have friends that you 'approve' .
What he should have done is to of bitten the bullet and told you that he was going to continue his friendship. To have been open about it and told you that you need to trust him.
I can see why he would choose not to see her with you - with your feelings about her now conveyed to your DH - how could he possibly enjoy a casual meet up with you sitting there and him knowing your feelings about her. ? It would be very uncomfortable.
I think your solution is to be open and honest and own your fears - whilst also excepting that they are based on past trauma .
'DH we need to talk about x. I know you still meet up. It obviously makes me uncomfortable - which has more to do with my past experience than present reality - but actually the secrecy is far worse and causes me great anxiety. So can we have a new plan here. ? I would much rather know about your relationship, meet ups, exchange of news than not. Jealousy is my issue to deal with but secrecy just exacerbates it . ?
Then walk that walk and seek some professional counselling to deal with the betrayal from your previous relationship- so that it ceases to infect your marriage. Take care . It's hard.