IMO,our society is obsessed with encouraging small children to separate from their families whether they are ready to or not!DS started nursery at 2.5 yrs,wept and def did not settle,lovely nursery rang me,tried again,again,gave up and then we restarted at 3yrs,-and I took him in at playtime, and stayed while that went on, for a good few weeks(the nursery suggested this).Then he was fine,is now 9 and somehow manages to survive the school day alone...!
It happens in its own good time!(and my DSis was the same,no nursery provision then,so she was with Mum full time - fully functioning adult now tho'!)