You're probably keeping the camera close to your face (at arm's length?)and using a wide angle objective (lens) . That causes distortions. (Most cameras have semi-wide angle standard)
The correct way to take a portrait is to take the photo from a greater distance with a normal objective. Pros use a small tele for portraits. (60 mm in Leica format, some even use a 75mm)
If the camera has a zoom, then take distance and zoom in, instead of zooming out and coming close.
In short: let someone else take the pic, your arms are too short for photographing yourself.