I am so, so sorry. Having just been through this a couple weeks ago the loss is still raw, and I know what a huge part of your heart Biscuit has claimed. Lots of hugs and love to you.
My advice that has helped me get through the pain is that I've given myself permission to feel sad when I need to. If that means that I sit silently for a few minutes, or cry a little when I'm alone, or even tear up when I'm talking with someone about Shadow, so be it. But the sadness comes and goes in waves, and in the end I can't help but feel such immense gratefulness to have had such a phenomenal dog in my life.