So sorry to hear of the loss of your mum, Daisy. Sending you a hug.
I also found my way here after losing my mum and again, I wasn't there when she passed away and experienced terrible guilt about that. It has never fully gone away. She was in hospital and I had spent every night with her for four nights as I knew it wasn't long until she passed and on that day, I left around 9.00 AM, exhausted, to come home for a few hours' sleep, but got call at about 11.00 to say I should go back asap and I got there at about midday. It was too late and she was gone. I still feel terrible about that. As Karena says, perhaps they do wait for you to go, so you don't have to witness the moment or perhaps the will-power to keep going fades when you leave them. I don't know, but as Karena says, you have to trust that someone kind looked after them at the time. It isn't unusual for this to happen. My gran passed away soon after we left to come home for a rest also. My dad had a heart attack and died alone too even though we were all in the house. It is horrible, but we can't predict when these things will happen and however strong the spirit, the flesh is weak and we do surrender to exhaustion at times.
In my case, like you, I was sad and felt guilty. Part of me was devastated also, both because she was gone and because I wasn't there, but I still remember that my overwhelming feeling was one of calm relief. I knew she wasn't suffering anymore and that was a huge relief to me. Perhaps that is part of the reason why you feel as you do. There is also the fact that, in a way, you already lost her fifteen years ago. A person doesn't have to have physically died for you to grieve for their loss and the body means far less than the loss of the person who inhabited it. Shock is no doubt another element of reason why you feel as you do. I suspect you will find days when you feel far less calm, but I think you will have known for a long time now, that this day was coming and in a way, having lost your mum so many years ago already, this just completed that and part of you will be happy that she is no longer suffering, just as I was.
You don't have to be in a state of distress and be crying all the time to be grieving. We all do it differently. I suspect the funeral may make a difference to how you feel. I find them almost unbearable. Others find them helpful. You will no doubt find your way to grieve as time moves forward. There is no right or wrong way to do it, so just be however you are and that will be right for you. It's a tough road and a long one with many twists and turns, but we do understand here and are here for you for as long as you need us.