He has NO business being on the same team as her. None. Or even at the same gym at all. No excuses. Right now, he may be saying it's only until the competition. Next it will be only because of xyz or until xyz.
YOU are the only one he should be "passionate" about in any way. YOU should be his ONLY "teammate." This is your right as his wife.
You can't nice him back to you. Being understanding, and letting him do as he pleases is not going to get him to stay with you and drop her. He is carrying on with an affair, either emotionally or physically or both. If it's emotional, it's bound to turn physical. If you appear weak or clingy, you stand little chance of keeping him. It's time for you to get tough.
Tell him he has two options. He needs to:
Option # 1. Call her this minute in front of me and tell her you have thought it over, and you value your wife and your marriage above all else. Therefore, there is to be No Contact between us anymore at all, ever. No phone calls, texts, emails, meetings, gym participation, (I am quitting it as of this minute.) Nothing. And then he has to stick with it. He should not allow the conversation to go on any longer, no matter what she says. There needs to be no consideration for her feelings, no "I'll miss you," or "it was nice knowing you," nothing. His tone should be business-like, not apologetic or regretful. Be sure he knows this before he makes the call.
OR he needs to: (your words to him)
Option # 2. Pack your things, making sure you take all you will ever want with you, and get out. Now. I don't want to hear that you need time to think or plan or choose. Go to a motel, an apartment, or to your wonderful "friend" that you just can't stop seeing. Just go. This minute.
There will be No Exchanges. No returns.
There are no other options. Which do you choose?