Polly - welcome to GN, but I am so sorry about the circumstances which have led you here. Family relationships can be very complicated but estrangement, especially from a child, is uniquely painful.
Regardless of the rights and wrongs of the situation, I think that your daughter is signalling that she needs some space from you right now. She has had some big changes in her life with her new partner and her baby. In my view, for what it is worth, you need to step back and focus on your own life. Hard though it is, you need to let go and not chase her, or you will drive her further away and diminish her respect for you. You need to be patient and give it time, so that you can forge a new adult relationship.
In the meantime, try to take good care of yourself and it might be a good idea to seek some form of therapy. Otherwise, our thoughts can overwhelm us and prevent us from enjoying our precious life. Don't tell her of your loneliness and put the burden on her to fix it, don't be needy or try to 'guilt trip' her into seeing you, or she will come to regard you as a burden.
Ultimately, we have to live our own lives, we cannot expect to live them through anyone else, least of all our children.
My advice would be to fill your life with people and things that make you happy. I know that Covid is restricting things at the moment, but could you plan a special trip, go somewhere you have always wanted to go, or take up something new - whatever 'floats your boat'. Maybe even consider an on line dating site or other ways of making new friends and companions.
I reckon that if you can become a positive, joyful person, a mother and granny who is fun to be around, your daughter will seek you out, as she will enjoy your company. Your daughter has a lot on her plate with her baby and juggling work etc. She needs you to be a support to her, not an extra demand on her.
I am sorry if this sounds brutal, but I am saying it because I believe it to be true and because I want to help you to move forward. You have raised two daughters to maturity, and you have the satisfaction of knowing that at least one of them is in a happy, stable, relationship with the joy of a baby. So give yourself a pat on the back and embrace the next stage of your life.