Welcome, and I'm sorry for your pain. I too lost myself in motherhood, and have had a brutal awakening in the last few years. If they told us when we were young women how it all might end up, how many of us would have had children I wonder.
I have seen more than a few stories on message boards from mothers who have single handedly raised the children after a divorce, only to have the errant father reappear when the kids are young adults and successfully buy the affections of the kids they have had nothing much to do with for most of their childhood. It seems so grossly unfair for kids to reward the parent who wasn't there while simultaneously turning on the parent who put their life on hold and stuck it out.
You wonder what goes through the minds of the kids, but I can only assume that immaturity is the major culprit here. These kids have no idea how much love and how many sacrifices it took for them to become healthy, successful adults. They get to adulthood and put their childhood behind them and take whatever good stuff is being offered to them regardless of the rights and wrongs of the situation. It seems often that the better the job we did, the less unfinished business they have with us and the more likely they are to leave us behind. Motherhood is a thankless job sadly.
I really hope that some way down the line, when your kids have kids of their own, that they finally appreciate what you have done for them. But for now, rebuild, rebuild, rebuild!! It is your turn now, and as hard as it is in the beginning, time to find pleasure again in things that are not child centred. And try not to fret too much about Christmas, and the happy families you see out and about. I think there are so many people who are struggling with some form of grief over Christmas, but hide it well. If you saw me out shopping with my husband and oldest daughter, you would have no idea that inwardly I am very sad that I won't be seeing my youngest daughter at Christmas.