It now goes against my values to flirt with other men.
!!! One may need affirmation in other ways, but the good feelings that come from being found attractive and desirable? IMO, those belong to one's partner and him/her alone.
As to your husband having what was likely a relatively innocent (if ego-stroking) interaction with this ex-student of yours? If it makes you feel uncomfortable or upsets you in any way, then he needs to know that and appreciate it. What he does with that information is then his choice. Frankly, I cannot see why she should feature in either of your lives in any way.
It seems to me that your argument was not about him continuing his "friendship" with this woman. (I don't think you said whether he wants to do this? If this is the case, then my advice changes) You were hurt that he had acted as he had - you saw it as him not respecting healthy boundaries. And he was hurt that you were checking up on him because he saw this as a lack of trust. (He probably also felt guilty after you told him about how you felt, which wouldn't have helped him see your feelings objectively.) Neither of you can control which feelings you have in response to this situation, they simply are. Sharing them is part of honest, open communication. It is understanding what to do with them and why they are there that matters. (For what it is worth, I would also have felt threatened and angry if I were you.)
However, the reality is that you need to trust him to be faithful in order to move forward in your marriage. Perhaps you already do so, at least in part. But, you also need to trust that he is fully invested in your marriage. That your comfort and happiness are important to him, even if it means respecting boundaries that he might not think are necessary. Trust like this is built over time by a thousand small gestures. It needs to be nurtured. Respecting your partner's feelings is absolutely necessary (and that works in both directions). Does your husband understand that you are not actually accusing him of having an affair? Were you hurt more by his initial behaviour or by his response when you told him how that behaviour made you feel? Your history possibly makes it difficult to have these types of discussions without one of you demanding that the other prove their love and commitment by showing trust first. The whole discussion then becomes stuck in a chicken and egg loop. Have you tried MC before? Or perhaps writing down your feelings so that you can share things like this more neutrally?
I hope that you can get through this together. It sounds as though you have both had painful journeys to get to this point.