Why am I so sad with the death of a former spouse?

I feel so sad and I am overwhelmed with the feeling that I had to look for some kind of support.
My former husband passed away yesterday due to COVID. He lived in California and we have an adult son who also lives in California. I now live here in the UK.
When my son rang last night to tell me his father passed away and what he had to go through to see his fathers’ body and how he can not breathe because of shock, I felt so helpless. I wanted to be there for my son but I am afraid to travel due to this horrible virus.
We were together for over 20 years but have been divorced for quite some time now. We were civil to each other for the sake of my son. I do not understand why I feel so bereaved by his death. I feel like I have lost someone very important to me, I suddenly miss him terribly and this overwhelming feeling of sadness envelopes me.
I keep on thinking of the past, I keep on thinking about my son and how he will cope without his Dad. I feel lost. I do not want to sleep, I keep on crying. I do not know what to do, why am I feeling this way? where can I go?

Thank you for your kind words Sheila, I truly appreciate it. This kind of grief is very new to me as I have not lost a loved one as an adult and I am trying to understand it.
I extend my support to you and thank you once again.

Dee X