i know how she feels. i always get this way after Christmas.
some things you can do next year :
avoid the whole scene of ripping every gift open at one time and just tossing paper and the item around for the next one. take it slowly; one gift at a time. once opened, remind her who the gift was from, and if they are present, have her say thank you right away. another good idea is to have thank you cards on hand and have the kids fill out a thank you card to that person right on the spot as the gift is opened. this will help them remember over time who gave them the gift.
another idea is to reduce the gifts she gets. i know, how horrid, limiting gifts? i have a friend who allows her kids to write a wish list of FIVE items, and ONLY FIVE items. if they want to add something to the list later, they have to remove something from the list. it works wonderfully. she also teaches her kids that even santa has to "make" the toys, so they "send" him money for them. this helps them to learn that gifts arent free, and that they dont come from nowhere... work and money is involved.
anyway, with the list, then you have something to concentrate on, and it can help to limit the items she gets.
ALSO; maybe she needs to learn a lesson in giving. this is going to be hard, and probably easier for you to do than her, but she needs to give away some toys. everyone knows someone who is poor or having a hard time. kids love 'new' toys, even if it is only new to them. have her pick out some toys that she can give away. even if it is only to a thrift shop or 2nd hand store. savers stores give you a 20% off coupon for dropping items off, and also they can give you a slip to fill out how much you think your items are worth, for tax purposes.
if its too hard for her to pick some things to give away, you yourself pick the toys out that she hasnt used in a long time. do this while shes not at home, or not paying attention. bring all the items to the table, and let her "buy" or pick one or two items at most from the table. the rest are going to the thrift shop or a needy person.
i hope this gives you some ideas about how to help. other things you can do is to have a child friendly new years eve party for her to look forward to. thats my saving grace; after Christmas, i get the "let down" but i still know that i can look forward to a good time new years eve. sure, then theres a let down after that, but try to have something to look forward to every weekend. one day a week, that your family can just STOP EVERYTHING and be together. play games. cook or bake. take a walk, go sledding, go to a park, whatever. this will help in general but especially after a big holiday like this. its things like this that i like to do for myself to forget about or get over the holiday let down.
anyway. its great that you can still provide a merry Christmas for your family, with lots of presents. but theres more to life than that. maybe your whole family can concentrate some funds to a worthy cause, or toys for tots or something, reducing what your family members are getting, but get that feel good feeling of giving.
and always remember, keeping up with the joneses will only make you more broke, and no THING will ever make her happy. TIME is what should matter. spend time with her. like i said, play games. read a book. take a trip to the library or someplace. do something with her.
above all remember: this is not your fault. her feelings are her own, not yours. you dont have to feel guilty for them. its just the way she is. i understand it because i feel the SAME way. for me its less about the presents and stuff, and more about the fact that my family has gone home now, and its so quiet and lonely here. i liked playing games with them all, and its so sad to have them gone.
see if you can talk to her and get any kind of sense of what shes thinking. dont judge, dont respond even, just ask her to tell you whats up and you will just listen. anyway, you are not a bad parent because she feels bad. i feel bad too and im the adult LOL
good luck