Astrological angles, the ascendant, midheaven, nadir, and descendant, are critical in delineating the natal chart, and when the ascendant, in particular, is on a cusp, it makes a real impact.
The ascendant represents one's view of the world and of the self, a result of early environment and conditioning rather than the innate sense of self.
If you were born on the day the Sun moves from one zodiac sign to another, you won't know exactly which is your true sun sign without having your chart calculated.
If your Sun is on the Cancer side, you are less shy and a bit more inclined to share your gifts and creativity with others. If your Sun is on the Leo side, you feel a bit insecure about your self-expression; it seems to reveal too much. If your Sun is on the Leo side, you are a perfectionist about your creative self-expression; it has to pass through the filter of self-criticism.
Even if your Sun is not quite 'cuspy,' you might feel as if it is.
One reason for this might be that neither Venus nor Mars ever strays far from the Sun.
Due to slight shifts in the Earth's rotation, the cusp's date varies slightly from year to year; there is no consistent date and time to rely on.
To find out exactly where your natal Sun is located, there are a number of sites where you can have a chart calculated for free as long as you know your time of birth. The degree on the ascendant changes every four minutes so it is essential to have an accurate birth time to work from in order to determine your rising sign and whether it is on a cusp.