Name what you CAN name. Don't worry about the rest for now. It will come to surface in time.
It might not be everything, but these are at least some things you can address I picked up on reading your post.
You have found a mismatch of expectations.
You were thinking swinging or kink playmate or open maybe? And he was thinking up to and including poly. You have named that part. So continue to discuss and clarify with DH what is on the table here.
You have found resentment and being pissed off about their constant interactions
via email/text/IM, however they can manage it. Well, what is "constant" right now? Every hour on the hour? Once a week? In your line of sight? Taking away from his being fully present when it is time with you? Define this one and come to agreement on something reasonable and acceptable.
Your heart knows you are secure and together. Does your MIND know it?
Do you have a framework
for how to be together? What rights and responsibilities your carry? How to deal in conflict resolution? Would having somthing there help ease your mind bucket (mental health) at this time?
Then I wondered...
Are you getting before before care and after care when he goes on dates?
That time to touch base and then reconnect with you so you feel valued as a person and not given a lick and a promise like you are some "meh" obligation or something? His duties to his wife? What about his duties to the household -- his chores around the house being executed well? Or are they being blown off to NRE?
Just my 2 cents
at this time = closed married polyship of 2 with DH.
Chronic patient = fuzzy brain at times. (If I make no sense in a post, just PM me and I'll happily try to clarify it later.)