That's a real cry from the heart, Msida. Everyone - well most Grans, anyway - feels like this at some time and that's without the added heartbreak of losing a husband or partner.
It probably comes to you in waves and another wave has just hit you. I'm so sorry that you're having a bad time. I don't think (personally) it's easy to do anything much about adult children. As you say, they are probably struggling in their own ways to make sense of life, the additional responsibilities, managing children, jobs, lockdowns, health etc., And who can find the extra moments needed to ask about Mum/Gran? Well sometimes it seems like absolutely no one. If you're very, very lucky you sometimes give birth to someone who cares always, but most of us just get average people who react in average ways in their average lives and would look quite surprised if you put the question to them "And what about me, how do you think I'm managing?" They're usually too afraid to ask, unless you are quite ill or have suffered a recent loss.
I think you just have to keep saying to yourself that when times change again - and they will - you'll work harder at making a life for yourself with friends and interests and outings and a routine which keeps you busy and makes you happy. Dog walking/owning/a cat for company/another hobby/exercise/social occasions/contacting friends of similar ages and interests/volunteering and bringing some smiles and real pleasure to those who are older or unwell or a lot more desperate. It can be done. I have a good friend who is testament to that.
She had not long moved to France with her husband. No children, no friends. He was still shutting down his business in the UK and was sort of commuting between countries. She started work in the garden, she unpacked and made their home so comfortable. They started to make friends, they loved the prospect of a new life. He went out on his bike before lunch on a sunny Sunday - and never came home. He was in his 50's and he had suffered a massive heart attack and was found on the grass verge coming back into their village.
I don't think I could have survived how she felt, but she did it. Once her grief was manageable, she tried so hard to get some routine into her life. Although she found mixing with people quite traumatic and often had to go home because she was fragile and emotional and hated to be like that in public. She joined lots of groups, made lots of acquaintances and then some really good friends. She has a real life again - not the life she would have chosen, she says, but a good life which gives her enjoyment and pushes her to get up in the mornings to have stuff to do, like playing tennis or going to the library, or the market or to help someone who needs that.
Maybe try not to rely on close family. Dare I say that way you'll avoid disappointment! They are always there in real emergencies, but for day to day stuff they are often far too busy and with limitations on their time.
I do hope your current mood lifts soon and you can see just small glimmers of hope can shine into your days. Posting on this Forum is always a good way of getting some company, some empathy and advice and allows you to feel part of something - when maybe you don't. So many have been in your situation. The Good Morning thread is a good place to start, or Soop's Kitchen, or the Night Owl thread if you find that nights are long and you just feel like a cuppa and some conversation instead of sleep.
Take care of yourself and keep safe. I wish you better days and happiness again. 
