Your beach picture yesterday was so very lovely. I am so sorry that you have to make this decision so soon but you are clearly doing it from a very great love for Riley.
I let my girl Scarlett go at home, on her couch, surrounded by her favorite people in the world. The entire day was about her. Friends were in and out all day visiting her and saying their goodbyes. My core group of friends was there at the end and we toasted her, loved her and then sent her to the bridge. I was holding her and telling her how much I loved her (and a friend was holding me). My vet was wonderful through the whole thing.
Claudia is right, you think you won't be able to handle it but you do. All that will matter is that Riley is okay, somehow, you forget about yourself. It will be devastating, there is no way around that but you will get through it.
You are in my thoughts and prayers.