Your relationship with your husband is changing, because this woman is now coming into it. His experience of who he is is changing because she is now giving him a lot of attention. He is responding to this by changing the way he gives you attention. Your feelings are one of the ways you are responding to this change in him that you are noticing and being affected by. No one knows what this change will bring to your relationship. He doesn't even know. The best thing you can do is ride the waves of emotion you feel and talk to your husband as clearly and constructively as you can whenever you need to.
Do you need them to slow down so you have more time to process? If so, tell him that.
Originally Posted by CheshireCat
I want to find ways to be happy (or at least comfortable) with any other romantic entanglements he may have
You used the word entanglement to describe the new relationship he has. What are you feeling that tells you this is the best way to describe his new relationship?