How do I fix my ceiling?

First I would check to see if there is some sort of moisture problem causing the paint to peel. After that, scrape off the loose paint, spackle, if necessary, and repaint.

Remove the peeling paint. You can caulk with a paintable caulk if the space is just a crack. If not then you will need to use the drywall spackle and fill in the area until it is smooth and sanded. Prime and then paint again. There is usually a layer of drywall paper between the ceiling and the wall and then spackled over top to give a nice sharp edge for painting. This has probably let go and this is what you are seeing.

Mine did the same thing and a builder told me I had a damp issue. Maybe you could find out the cause of this peeling first. If you have no damp you will just need to sand the old paint off and paint again.

Once you know there's no damp, you could add crown molding at the junction of wall and ceiling.

What might be happening is the drywall seam tape that runs along the seam is coming loose. I have a small section of wall that did this in the winter and then as it got warmer in the spring and into summer I couldn't see it. I eventually repaired it.https://homeguides.sfgate.com/retape-wall-ceiling-joints-26573.html