Sorry to hear of your last 6 years of suffering. Going through any kind of mental or physical illness is no bag of chips. Applying for SSDI/SSI is another whole process, but it sounds like you did get aprroved for SSI (not sure about the denial for SSDI). As far as I know your back pay is yours to spend. Now if you go buy a BMW or something crazy then they may look at you and question things, but xmas gifts for family and friends, go buy them something if you want to. Go to the dentist and eye doctor too, but if you are going to get medical coverage soon (which you should being on SSI) why pay out of pocket, unless you want to go to your own doctors and not the ones medicaid pays for?
I was told as long as your bank account for SSDI/SSI $$ is kept seperate from any other $$$ you have and they do not ever comingle, then if you have creditors or debt collectors looking for you, they cannot take your SSDI?SSI account. You can also get one of those Direct Express Debit cards, has a MasterCard logo on it. They can deposit your monthly $$ right on there and you can check your purchases, balances right online on their website. Your call.
Try to enjoy your Christmas and spend a little if it makes you feel good!!!