OP I know this is a huge shock probably to you all. You sound overwhelmed, exhausted and worried for the future a lot of this is rooted in how much you do for your DD.
I mean this kindly but you have 2 grandchildren now not one. Both your children need you and both your grandchildren need you. As it stands it very much seems you are so over committed to your DD and her child you cannot support your son and his child.
You need to step back a bit from your dd now and let her take on more of the parenting of her own child and offer your son some support. If you dont you risk pushing your son and his child away. Your son and his child need you now and your dd needs to stand on her own 2 feet a bit. She isnt a child. She is a 24 year old woman who made an adult decision to have a child.
In my dhs family MIL helps her dds with childcare. She does everything you do but only for her DDs. Her sons and there kids don't get that help or any really at all. She is so focused on her dds needing "help" it has come at the expense of her sons and there children.
My dh feels totally let down by her because we have struggled for years with no help whilst watching the females in his family getting endless support, frequent weekends off, financial help, free childcare etc. My children have barely any interest in my MIL now as they know she always prioritises her dds and there kids. Do not let this happen in your family. Its toxic and unfair on your son and his child.
Right now your basically saying you will help but only your dd. You are overwhelmed by the level of help you give her and your other commitments so now this unexpected new grandchild has arrived and you need to reassess your priorities. Your son and his new baby need to be higher up the list of those properties at the moment than your dd. You have helped your dd for years now offer some of that help and support to your son.