My beloved Coco (only 10) crossed the rainbow bridge on Tuesday and I'm struggling

She was my world. She did everything with me. Slept with me, ate with me, went where I went, She was my little shadow. She was always happy, and loved everyone. Monday she was her usual loving self, and then at 11pm on Monday night she had trouble standing up. I took her to the vet and they had no answers. I took her home and held her in my arms until the regular vet opened up in the morning. I took her to the vet and they ran tests and they couldn't find a cause. She had started declining so they kept her for observation but later on in the day they called and said there was nothing else they could do. I chose to not let her suffer anymore and it's killing me. I hope she's as happy in the next life as she was here with me.