Nanastrawberry - first, I want to send a big virtual hug to you and to say that you are not alone and you certainly shouldn't feel ashamed, although I know that many estranged parents do. Estrangement is surprisingly common in our more fractured society, but it is extremely painful and the grief - it is grief - can be overwhelming at times. Most of us on this site have experienced this and may continue to experience it, whether they be estranged parents or estranged adult children. You are among friends here, people who understand exactly what you are going through, although each person's pain will be as individual as they are.
Many estrangements are not permanent and may be triggered by a number of different factors. It is inevitable that you will reflect, search for answers, but sometimes it is hard to find them. In some ways, it is like a surprise cocktail; you throw in different ingredients, personalities, situations etc and that is the result. And you need only change one ingredient to get a wholly different result. But you can drive yourself mad by spending too much time ruminating on it all - night times are particularly difficult, in my experience.
I understand the desire to take action, to want to fix the situation, and why you are considering legal proceedings to try to force a change. However, I would take deep breaths and reflect very carefully before you go down that road (and I say that as a retired lawyer). Grandparents have limited rights in the UK and the process is not only difficult and expensive, but you run a very real risk of exacerbating the situation, as litigation often tends to polarise positions.
My advice, for what it's worth, is to be patient and to keep busy, take care of yourself and try to do things that make you happy. I also had therapy, which I found very helpful. Don't put your life on hold until the estrangement is over. There is always joy to be found, even when times seem dark.
for you. X