First off, I am so very sorry you have to go through this
It is so difficult either way. We have lost 2 dogs in the past year and a half and both we had to euthanize. I know you already went though it with one of your dogs but I'm going to explain everything I went though both being there and not being there.. maybe it could help someone else who has never done it before to make that decision.
First was Ally, our long haired shepherd. She was only 4 and developed hip dysplasia.. it got so bad she couldn't stand up by herself or walk on laminate flooring without her hind legs going on on her. No one in my family was strong enough to be with her while she got put to sleep so I did (ahhh I'm already starting to get teary eyed thinking back to it) It was REALLY difficult, but like you, I would have felt like crap for the rest of my life knowing she was in there dying alone. Though I'm left with memories that I still think about all the time, some not so pleasant, I think I would rather have been there than not. I got to comfort her in her final moments and she got to see a familiar face in an unfamiliar room.. it also gave me closure.
Our other dog, Spot, who was a pitbull had to get put to sleep a year ago. One day my mom called me and told me she was taking him in and didn't really give me the option to come or not. I still feel guilty for not going, though.. but I also don't have to live with some sad memories of watching him pass on. My mom did stay with him the whole time so I don't feel as bad as I could have, I personally would feel bad if he was alone. I was happy when I found out she stayed with him.. but it did take me a long time to accept he was gone since I wasn't there.
Soon we will be having to put our cat to sleep (he has kidney failure) and I am faced with the same dilemma. I've had Leo for 16 years and he was my best friend and helped me get through some tough times growing up.. I honestly don't know if I will stay with him or have to leave the room when he gets put to sleep. As difficult as I know it will be, I think I will end up staying with him. He's my baby and I don't want him to be scared.
So I guess what I would suggest is if you are emotionally strong enough I would stay with her especially if no one else is willing to. That was the situation I was put in with Ally and I don't regret it, as hard as it was. Stay strong and good luck with your decision