The millstone was a large and heavy thing used for crushing grain into meal or flour.
A milestone is a marker at that distance from another point, often another such stone.
In the case of the former, it has sometimes come to mean a thing that drags one down, the proverbial weight hung around one's neck, say.
Milestones metaphorically show accomplishment. Passed another one along the road of life, which could be good or bad, depending.
Getting rid of one might help you reach another ...
Did that yesterday. Got up at the electric blues jam at The Lehrer, stood in front of the mike, and led a couple songs. Never played the uke standing in public before. Hadn't intended to, was just going to drop round and listen, but I took the uke along, just in case. Got coaxed into it, and while I skipped a verse in one song, there were enough instrumental breaks nobody missed it. Voice was okay, even managed to get that blues rasp when I wanted it.
Another long-carried millstone put down and a milestone reached. At this rate, Carnegie Hall won't take more than another fifty or sixty years ...