*Sexually explicit *
The average woman's vagina from opening to cervix is only 5 -7 inches so it's entirely possible the OP is having problems.
When a baby is born the cervix opens, it doesn't increase the length of the vagina only the width is this malleable.

Options, find a long term partner, take your time, prepare properly.
Try not to get overly forceful or aggressive when your about to ejaculate.
Try positions that don't encourage full penetration.
If your with an adventurous partner you could try an*l for full penetration. But do use protection. And again lots and lots of preparation and lubricant.
All the best, and good luck.