Hello JustineW, thanks for your post and welcome to the site.
Even though your son has moved out this relationship is still going to affect him, especially if you start to live together and your daughter might worry that all your attention might be concentrated on you with him, but realise you would be giving her all the support you can.
I know from experience, with my ex living with a chap 20 years her senior that my sons weren't happy when she brought him to family occasions and this included myself, but I had no say and talked to her on the phone.
Now he is in a nursing home and although this may not affect you now but later on in life.
You want to be happy in life and that's what you deserve, and perhaps you could write down a sequel for the day so that your daughter feels much happier.