No, it's definitely not true that some women can only reach orgasm from penetration. The majority of women do not climax through penetration alone.

You don't mention what you've tried when stimulating your clitoris. I would suggest picking up a vibrator for yourself and setting up the time and mood to use it. If you find that erotic fiction gets you a little hot and bothered do some reading before trying to stimulate yourself.

 

If you need to, use a mirror to see exactly where your clitoris is and then apply the vibrator to it. You may just need to relax a bit as well. I know that for myself sometimes getting off is really difficult and it's that much harder when my head is not in the right space.

I hope these suggestions will help out a bit. Don't give up!