I’ve found this as well with faux painting and it just takes practice. I would tape off the ceiling so that when you do the wall edges you actually apply the paint a little onto the tape, thus making it seem more natural and less like a hard line. In the corners, start a few inches on one side and blend it onto the adjacent wall. If you get some momentum, it will appear to move from one area to another and not look so cut off around the edges.