I was recently approached (via a messager service) by a friend for some counsel, since she was troubled by a possibility that her feelings after a significant other had a late-night conversation with a platonic friend might have been unfair or unreasonable to the others involved. She found my response very helpful, and upon request, agreed that I could share my words here, slightly edited.
Here, then, is what I said to her:
"Here's the thing: feelings happen whether they are reasonable or not, especially jealous feelings. And we cannot usually control them. So, what matters is how we respond to our feelings. Do we let them control us, or do we make the effort to carefully consider them, and try to let them go if we find that they are unfair or unreasonable?
Another side of not letting feelings control us is that we shouldn't feel bad or obsess over it if we have had feelings that we find to have been irrational.
Now, you are trying to figure out how you "should" feel. Since I don't know all the details, there are big limits on what I could say, even within the limited range of what anyone can say about what feelings "should" be. Still, I can make some general points, and maybe pose some questions you can consider to help yourself.
Let's see... you mention that you have agreed to it being healthy and acceptable to continue to have platonic friendships with others, within "boundaries". [I did not say so at the time, but when people try to forbid romantic partners from having friends, it is a kind of emotional abuse, so the agreement mentioned is both healthy and normal.] If they had violated any agreed boundaries, you probably would have said so, and would be much angrier, so I conclude that there are no such problems here. If so, then it is purely a matter of your feeling hurt about it.
I think it is understandable -regardless of rationality!- to feel hurt or jealousy if a significant other spends a long time talking to someone else, or even simply at a time one might feel belongs to them... late-nights are often private time for intimacy, which means time usually reserved for significant others. It is a more rational feeling if, say, the conversation with another is taking away from your own time. On the other hand, sometimes a platonic friend may have a special need to talk, such as an emotional crisis or other problem, regardless of the time. So that consideration means that it's good to try to be tolerant and forgiving if a friend needs some of your significant other's time.
Then there is the question of fairness... for example, you are confiding in me and asking for counsel. How would you feel if your significant other was to feel hurt by this? Or, for that matter, if someone on my side was hurt by my talking to you? You would probably think that was unfair, in both cases.
Again, feelings aren't necessarily rational or fair, so the real question is what we do with them when we have them. But then, while some feelings simply come up in us, we can also make decisions that are or else involve feelings: forgiveness, love, tolerance, etc. If we feel hurt, whether or not it is fair in a particular situation, we can counter it with the choice to forgive, to tolerate, or simply to love, sometimes unconditionally.