That is very painful. If it helps, at all, we don't know why people do these things, and often they don't really know themselves.
Would it help if I gave you an "alternative" explanation? People often (not always, but often) try to do the "right thing" but there is so often in life no easy way of deciding what "the right thing" is, other than by outcome.
Maybe in her head, your grandmother had a vision that she was, somehow, going to give you the thing you most wanted: a wonderful mother, a wonderful relationship with her, and through that, magically, a wonderful childhood, delivered back to you, like a beautiful jewel, that you could gaze into, and be magically filled with childhood happiness.
Likewise, your mil.
Of course, that didn't happen.
There is also a touch of arrogance and selfishness in people, sadly. They often mistake their powers to put things right, and to understand things - they over-estimate. And they also want an easy life - they want to take the path of least resistance, and to not have to do uncomfortable, unpleasant things, that may make others judge them harshly, or think of them badly.
So it's possible that your grandmother was alarmed at failing, herself, as a mother, or thought she knew best, or was just feeling uncomfortable with keeping confidences. Maybe she felt responsible for what her daughter did, and felt she had to, somehow, make up for things she failed to do earlier.
None of this makes things easier for you, but it may make it easier to think of/live with if you can believe it isn't a pattern of people letting you down/abandoning you.
Also, you know, what comes across so clearly, is that despite everything you really are someone you can rely on. that is such a good, good thing.