Blueangel, I had goosebumps reading your post as, at least the beginning part, could have been written by or for me.
I too had been heavily involved in helping to look after my Grandchildren until recently. I used to help every weekday, from 9 am to 7 pm. I went on holiday with them every year but was told this year by my daughter that they wanted to go on their own. Before anyone has a go at me, I did understand this.
However, after their return, I have been been excluded more and more, have been told not to go to the childrens' birthdays (although I am still expected to buy lots of expensive presents) and since the older children have returned to school, my daughter told me that she doesn't need me to help with the little one, aged just over 1 year old. She then said that as she does't need me, she doesn't want me and I am only to go 3 times per week after school, to help with the difficult time while she cooks and the boys are tired, hungry and tend to fight and argue. I do appreciate that I am lucky to see my Grandchildren at all and that 3 times per week is a whole lot more often than many people get.
However, I have been bereft, have no friends and am very introverted, so as Granless has pointed out, it is difficult to join groups. My daughter was very cross with me when I told her this and that I didn't fancy the bingo & board games group in our village anyway.
I wouldn't mind volunteering at the Grandchildrens' school, but cannot bring myself to ask about doing so for fear I might burst into tears while explaining why I want to do so. I spend almost all day every day in tears as it is and tend to just hide away. I am 66 and a widow.
Good luck, Blueangel, I hope you find something to give you a purpose in life.