There may be another answer than these two alternatives. It is often that people will choose someone similar to a person they’ve chosen in the past. Your fears are not all illusions. He was hiding information on you, he didn’t respond back to you on Christmas, and this woman has obviously become closer to him than other relations at work.
Whether or not he has feelings for her isn’t as important as the fact that his behavior bothers you. The sending of itself is enough for the two of you to have a discussion. It isn’t an all or nothing situation. You want him to know that his secret-keeping, and lack of responsiveness on the holiday were, in and of themselves, not OK. Stick with the facts. This isn’t a matter of trust as much as it is a matter of lying through omission. When he’s been with her—he’s not been truthful with you about it.
That doesn’t mean you have to bolt on the relationship. But it does mean you need to be clear about what is and is not okay with you. If he knows you have an issue with jealousy, then he’ll need to learn the behaviors that activate it.
Have a discussion, and if you feel you need some help you can find a therapist nearby at the ‘find help’ tab at the top of the page, or perhaps from this organization.
Wishing you patience and peace,
Proof Positive Blog @ PsychCentral