I am really sorry for your loss.
As someone who lost my DH in his mid 50’s can I bring some insight to what your DM might currently be experiencing.
At 18 months I was still very much awash with grief and simultaneously struggling to navigate the concept of forthcoming retirement and old age alone.
That just wasn’t the gameplan we had envisaged. The work of building a life together, bringing up a family etc. was fading into the past and we were planning more time for us. Time to travel and relax a little more with new hobbies, a new home etc.
Instead, I was thrown into numerous financial and practical issues, all the time grieving the person I had shared so much of my life with and the future I had expected.
All that at the time when the world, family and friends had started to move on and I felt left behind.
I was a mess emotionally. I got up every day, tried to plaster on a smile, attended to the essentials and just coped minute to minute, day to day.
18 years later, I have a full and happy life. I lost friends during those early years, couples who couldn’t cope with my singledom, but now have many, many, more. I have moved halfway across the country to live where we once planned to live together, go on holiday several times a year with friends and spend as much time as possible with my family.
I urge you to give your DM time. Bereavement is a winding road and some people require longer to come to terms with their loss (the third year was the worst for me). Talk to her about what she is feeling and tell her you are missing her involvement with the Grandchildren. Make plans for the future which involve her and encourage her to feel that she does still have a future to look forward too. Even if it will be different to what she once envisaged.
When you lose a partner you lose a massive part of your past and your future dissolves in front of you. You may find that by helping her to create new plans, you actually end up with a closeness that you would never otherwise have experienced.