I am so very sorry.
We've had our share. But our last two dogs were with us through 30 years of life, which is somewhat amazing.
Nakaya was the dog whom our kids grew up with. I'm suspecting much like your Ren with you. When she got the big terminal tumor, the kids came over to say good-bye, much emotion, many tears, because she was as much of the family as any of us.
Quin was our empty-nester furfriend. She was the other half of that 30 years. In that time the grandkids had grown up with her. Terrible sadness on her departure.
And here's the really tough part. You don't want to take them on that final journey too soon - but more than that, it's important not to wait too long. And you can second-guess yourself and your humanity into waiting too long. But it's not the best for you and your friend.
In its own way, having these furfriends has questioned my own attitudes about human life and death. Must we hang on so long? What would be so bad, if no hope remains but a slow, pain-wracked. and drug-addled slide to the end, to cut that process short surrounded in comfort by your loved ones? What is so wrong with that?
So sorry for your loss. Enjoy the good memories of Ren that you've grown up with. They will emotionally nourish and help fulfill you and ultimately overshadow the instant grief and loss that you must feel.