Welcome to the forum and my apologies for the long delay replying to you. It's not the usual way but does sometimes happen.
My condolences on the loss of you mother. My mom died 20 years ago and I still miss her very much. Now and then I find myself saying, "Oh I must tell mom about ...". Then I remember and cry. My mom lived in the UK and her death was unexpected. I returned for her funeral but it was not the same as being able to say goodbye. I cried for months every night when I got home from work, and sometimes vanished into the toilets at work if I became too emotional.
It's not easy to lose anyone we care about but moms are a special breed I believe. They are the people who nurtured us from baby to adult, rejoiced in our successes and consoled us in bad times. I'm sure we would all have a very long list of things we learned from mom.
Grief is personal and takes a different form for everyone. I think you are saying you are finding it difficult to grieve and that you should be able to do so. Do I have that right? For me mom's death was shocking in its unexpectedness unlike my sister who died from cancer and who we knew was dying. I still find it hard to grieve for her but I did go to the UK to see her a few weeks before she passed away. How much difference that made I don't know.
So a couple of suggestions that I have found useful or know others have found useful. Can you arrange a memorial service for your mom? Not necessarily in church, just a place where the family and friends can gather to remember her life. You could make this a function at your home, BBQ or afternoon tea. Get out the family photographs and talk to each other about the funny things that happened in your lives with mom. Talk about how there is is a mom shaped hole in your lives and how you can fill it in a positive way. In fact talk about anything and everything. Don't be afraid to show your emotions or tell others not to cry. Missing someone and crying for the loss is natural and also a tribute to the place they had in your life.
I have written this quote several times but I find it valid. A friend was told after the death of her son, "You will never forget but the times between remembering will get longer". I have found this to be true. I will not forget mom and sometimes the memory is painful, but we can continue with our lives and often smile at the memory of the person we love.