Hello ... you are not alone with these feelings, this angst fear and torment you experience when you grapple with the thought of death are experienced by many people mostly after a life changing experience such a the death of someone significant in their life, or a trauma or some sort which makes them think deeply about their humanity, and how fragile we are in the world. It is existential angst....and brings serious feelings of dread which cannot be just whisked away, they are haunting. I do so feel for you going through this right now, I had the same experience much much later in life though - i was in my 30's when my brother died suddenly and over the next few years I struggled constantly with everything you describe here. Death and trauma in our lives can overshadow everything else, and remove all our barriers, defences and the beliefs we constructed around ourselves to keep ourselves safe. Sometimes all this is necessary, all this graplling, to try to find some meaning, a philosophy for living - which may or may not involve 'religion' - I understand what you say about that as well.
It is the beginning of a journey for you, it is hard to live with all this fixation on death but it is something that has found you, you have not sought it. but while there are many many people who will not udnerstand what you are talking about, there are also many many who will understand completely. You are not alone. Google 'existential angst,'you will find information on this, check out the James Park museum of existentialism.....it is a philosophy for living, not saying it has al lthe answers or even some of them, but it is a good start & in your searching to make sense of your feelings and anxiety, you might find something useful....I know I did. Many philosophers and writers have dealt with the subject over hundreds of years...all of them searching to get to the bottom of this deeply felt anxiety. Yalom is another (current) writer, a psychiatrist, and I think he talks about the 'givens' of life, one of which is te certainty we will all die, and the angst that is related to that he believes is at the root of ALL anxiety, though it is only recognised by some as it doesn't always come to the conscious surface. These feelings can be dealt with in therapy or counselling - counselling would be very helpful to you, I am sure, if you get the right counsellor, see if you can be referred for help as talking all this through can help release some of the pain you have as well as help you deal with all the grief you must experience from your mum's death.
Good luck in your searching, take care of yourself, know you are not alone, develop your own philosophies through life to help you cope with this. I am sorry this is such a long ramble and hope it makes sense to you. I just felt I had to write something when i saw your post as it reminds me so much of how I was all those years ago.