My heart breaks for you. I know it's an extremely painful decision to make and a very personal one.
My husband and I knew right away that we wanted to be in the room with my Brody when the time came but my daughter who was 20 at the time was pretty strong up until we made it to the vets and he explained the procedure to us and she started to have second thoughts. Brody grew up with her so she was very distraught over her decision as well. She told me she didn't think she could be there because she thought the image would be in her mind for awhile and partly because she didn't know what to expect. She struggled with her decision for a bit and decided to stay in the room. I think for her she managed it by thinking of all the times Brody never left her side. Brody was always there for her and she didn't want him to be in a strange place without her.
Like I told my daughter, there's no right or wrong. It's a personal decision.
We're all here for you.
I wish you peace with whatever decision you choose