Thanks for the advice, everybody! She's sad but handling it well. I'll ship presents, which I always do, and Skype in for the present opening.
The surprising part of the events is when my siblings found out I wouldn't be home for Christmas but I'd be going to Grandma's for Thanksgiving is that they all changed their Thanksgiving plans to meet me there (car trip distance for them.) We aren't always the greatest at staying in touch but it's so nice to feel loved
Also, on the "softening the blow" aspect, I think the important part here is that your mom may need to develop new expectations about you -- the single person -- being the one to travel great distance every single year. I went through this myself.
For example, just because you're in a 2 bedroom condo right now doesn't mean that in 18 months, when you're "due" for a Christmas visit, it's out of the question that you'd be in a serious relationship
and sharing a house. Something major might be ongoing with career or studies.
So it's about an attitude adjustment. Some parents need time for this; others (like mine) respond better to hitting it head-on. I did this with a heartfelt but direct email, confirming how much I love "Christmas with the family", but if I can't do it, then they need to accept this at face value. Applying pressure won't change the result, and it just creates awkwardness.