Hey Dubious, welcome. There are many people for whom this gets easier over time. It sounds entirely possible that your wife is merely infatuated if there is no real intimacy between them, but the fact that love was forbidden probably made it all the more irresistible for her to feel this way. This in no way has to end your marriage as long as you still feel like you can trust her and she has respect for you and your feelings, even though she's taken this painful step in order to honor her own feelings.
Some questions --
Have you met the play partner? Do you know him as a person. Has she told *him* how she feels?
Why do you think this feels so impossible for you? Is it an issue of jealousy, fear, feeling not important any more, all of the above?
Mags is right, get some sleep and good food in you above all else.
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.