I'm so sorry for your loss. I lost my 87 Yr old mum almost 3 months ago. I still can't process it and I can't stop crying. 

You may feel like you let your mum down but by the sounds of it, she wanted you to leave as she didn't want you to see her struggling. 

I stayed with my mum for 3 days solid. The last hours will haunt me until I leave this earth. To see her struggling for her last intake of air and the noise was just awful. I wish I had not been there at the end. My mum seemed to be wanting me and partner there. I stepped out the room when the death rattle became too much. I spoke to an amazing nurse who just sat and held my hand. I took a deep breath and went back in and stroked her hair. I felt her take her final breath while I was holding her shoulder. 

I have nightmares every night and I wake up hearing her breathing. I would take salvation in knowing your mum wanted to go in her own way and didn't suffer. 

Take care x