I want to decline an invitation due to resentment -- but not burn bridges. How do I recuse myself?

At Christmas, my mother, who is almost eighty years old and seems even older—she had managed to alienate my aunt (father’s sister), my nieces (who didn’t want to visit because my mother was always berating and yelling at my dad—who cannot help his condition) and me— plus, my mother vehemently hates my girlfriend—whom she’s never met (knowingly) and never wants to meet.

Three months later, my Mom (who, along with my dad, are in bad health and have mobility problems and don’t get out much) has made up with my aunt (which I am glad) and my nieces seem to be coming around. My mom wants to get out and go to a restaurant with everybody. I usually drive them,

My mother has met my girlfriend twice and liked her (they got along very well) but didn’t know who she was. I didn’t tell my mother. Trust me, this would have been disasterous had she known.

My GF and I are very close and want a future together and she desperately needs to be around people and a sense of family. But of course my GF was not included in the invitation as usual.

At Christmas, my GF was homeless and we were pretty much living out of my car and hospital lobbies, blending in with the crowd. She spent Christmas homeless and alone while my extended family celebrated. And I did some phony, PAINFULLY phony prayer at the meal about being thankful for sharing and having a family and home.

I don’t mind driving my parents to the restaurant or getting my aunt to. And in my poverty I rarely turn down food and atmosphere / respite. But I really don’t want to sit there while my GF has nobody and I will have nobody at the restaurant. In a way I am making a stand but don’t want to make a “scene” or burn bridges. I simply do not want to be there “enjoying myself” among family while someone I love is excluded, out of hate. Unbridled, unknown, hate of someone my mother doesn’t even know.

I don’t want my GF to be alone and I will be alone, too—

At least my sister’s family know and LIKE my GF. My father knows little about her but might if it weren’t for my mother. My aunt won’t give a “stranger” a chance but probably doesn’t hate like my mother.