DH was the 4th child his bio mother birthed, and he was put up for adoption as a newborn but was named by her. She went on to have 2 or 3 more children, and married DH's bio father. He was adopted to another family.
When I met DH, friends had made efforts to contact his bio M with small success. She is still alive but says it wasn't her but her sister who birthed DH. Older half siblings/aunt/uncle clearly remember her leaving for hospital.
From what we can find out, she isn't a very nice woman, is manipulative and has the remaining (adult) children absolutely terrified of her.
So I have a little bit of insight how you are feeling.
There's no easy answer, or answer at all really. Had your Mum been alive, if you had asked her, you still might not have received ''the right answer''.
Your Grandparents might well have their own theories but to protect you, might have glossed over some of your Mother's less positive characteristics.
I also have a friend up the road who admits she doesn't have a maternal bone in her body and thought things would change when she gave birth. It didn't and her son grew up with his Dad, she rarely sees him.
I don't know how all of that sits with you, and if its enough or not, but please be 100% sure it was nothing to do with you or the little person you were.
This was something well out of your doing... and growing up with your grandparents was most likely the very best way for you. Try and park it up for a while and find joy in your life, one day you may very well have your own little family unit, and I hope it brings you some peace and love to have your own family, as it did for my DH to have ours.