I’d suggest that you ask your daughters to go out with you someplace else for the talk if your W is not receptive to doing this jointly. Below is a suggestion...you can change it as you see fit.
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D1 and D2, I wanted to talk to you two about Mom and I. As you may know, Mom and I are having our struggles in our marriage. We do not know what will or will not happen. Please know that Mom and I love you very, very much. That this isn’t your fault nor your responsibility at all. It is between Mom and I as adults. You need to know that there’s a lot going on with grandpa (FIL) and grandma (whatever names you use for them). That can be quite stressful to deal with and Mom is on her own journey that we may never understand fully. She needs to walk her own path. I love Mom and I know you do love Mom too.
Things are going to be different and we don't know yet what is going to happen, but we will be fine. I know it's hard not knowing. For now just know that I'm here and if you have any questions or want to talk I'm always available.
I love you both a whole bunch! [however you want to close the talk...
