It hurts so much .. how lovely that you saw his begining and end. My beautiful Cassie lost her life to cancer 5 weeks ago.. the vet came to my house so she was with me in her own home. I didn't see her birth .. she was a rescue dog .. but she was my constant companion for 9 years .. my best friend. I'm struggling without her because she was a joy, but so grateful to have had her.
So sorry for your loss...
All we can do is take comfort in what we offered them and what they gave us in return.
I lost a "work pup" to cancer. It was untreatable and it was aggressive (rapidly progressing). She wasn't my personal dog, but she was my "baby".
Through the pain you had valuable time with Cassie and she had valuable time with you. That's what really matters.
Sending hugs..