I'm so very sorry.
Love knows no boundaries or species; my vet and I both cried when we euthanized my 20 year-old shephard, Sugar, who had bladder cancer.
Mostly, the best thing for grief is time; giving time to grieve, then as the years pass you can move through your life and just cry or smile a bit every so often as some joyful memory comes up. You move the sofa and see one of her chewie toys or notice that the flowers over her resting place in the backyard are thriving, that kind of thing.