I don't know if I have any answers, but I would like to give my perspective.
Your kids sound like they are still fairly young adults. They are busy finding their way and building their lives. My husband and I did that too. We were so wrapped up in each other, and our jobs, and learning to live on our own, exploring, wanting to be independent, etc.....and we thought our parents would "always" be there. There is a separation that needs to occur in young adulthood.
Unfortunately, we were mistaken in thinming they would always be around, there would always be more time...
my mother passed away when I was 27...just before I became a mother myself. It wasn't until I became a mother myself that I really realized how much I still wanted and needed my own mom. I felt I had a new understanding of/for my parents and everything they experienced.
A year later, my father-in-law passed away suddenly and unexpectedly. My MIL, whose children and husband were HER LIFE, became severely depressed and withdrawn. She lasted only 4.5 yrs before choising to end her own life. She was in an immense amount of pain, and no one realized quite how severe it was until it was too late. So, by age 33, I only have one living parent. My children, 1 and 5 at the time, have only one biological grandparent still alive. We NEVER thought it would be like this. Never.
The point is this...cultivate your own interests, find things you enjoy, spend time with friends and family you love, take care of yourself, finally realize its ME TIME after spending so much energy and time on others. Your children will find their way. They will build their lives. They will come back. They still love you. Don't turn your back on them. Continue to be there for them, just realize their needs are different now. Be proactive and positive and happy about the time you spend together...no matter the amount. If visits are happy and positive and loving, they will come back more often than if they are sad/mad/jealous/negative. And someday, they will realize how much you gave to them. How much you did for them. How much you gave up for them. And they will want to talk more, and visit more, and bring their kids to see grandma.
This is yet another phase, Mama. Perhaps one of the hardest...