Thanks again for your thoughtful replies. I feel like I have a lot to think about and process.
I've realized that part (though by no means all) of the problem is due to my insecurities as well as past hurts and distrust, and it's sort of been exacerbated by my husband's and his girlfriend's behavior of late.
And my partner has decided recently that he wants to talk to me more about other people. I have no desire to limit or control anyone's behavior, any more than I want that done to me, but I'm a bit overwhelmed by everything that's been happening. Between my husband's behavior and my partner's insistence that I talk about the people he's interested in (I do tend to just sort of shut down when he mentions these things - I'm not angry, and I want him to be free to do what he wants, I just have no idea how to respond to him talking about other women) I'm just sort of sad and panicky. I've been feeling like I won't be important to the people I love anymore and that my needs and wants will be ignored.
So, lots to think about. I just keep thinking that the only person I can control is myself, and I'm going to take care of myself and choose to be strong and as calm as I can. I will get through this, and probably be better for it. I had a lovely night out last night with my partner, which helped immensely.
Other thoughts or advice are of course very welcome.