Dear Emm14, lots of advice here, some you may want to take and some you may want to leave. Are you actually more upset, rather than jealous?
I try not to be jealous (but was initially often VERY upset) when in our case, my SiLs mother has decided that she is the ‘real’ granny and therefore the one to do the majority of the babysitting, etc. She announced on several occasions, even when I was actually in the room (!), that she is the ‘best’ granny in the world. She has even physically pushed me out of the way to get to one of our ‘mutual’ grandchildren first!
I have said several times that in my view children need as many grandparents, and as many people to love and care for them as possible, but I’m afraid that this falls on deaf ears.
It sounds as though you had a wonderful Christmas with your family and grandchildren (as did we, which was marvellous) and now they have gone to their other family (as have ours!).
I have found some online Mindfulness exercises to be very helpful, and have also taken advice from other Gransnetters, which has helped tremendously. Hope you are feeling better with lots of lovely holidays and Christmasses to come!