I had similar problems when I was yonger. I tried and tried again to use tampons (and insert other things) but it hurt and simply didn't work. I was convinced that I had vaginismus or something along those lines. When I finally found the courage to go see a gyno (I was 21!) I found out that I had a microperforate hymen. I ended up having to undergo minor surgery to remove / open up the hymenal tissue. That was 5 years ago, and I haven't had any problems since!

Anyway, I have no idea if this is the case for you. But I really want to encourage you to go see a gynecologist. I wish I hadn't waited until I was 21.