What size of tampon are you using? Did you know that the ones marked for lighter days are lots smaller? When I was first having periods, I thought they were all the same size but just different absorbencies, and was trying to insert super jumbo ones (because that was the kind my mom kept on hand) and had very little luck. Only found out that the light absorbency ones are about the size of a pencil many years later, which is a shame because they would have been much better for trying initially. It's possible that the hole in your hymen that releases your period flow is too small for any tampon, but it might be an OK size for the smallest tampons.
Hi. I actually had this problem and it turned out I needed surgery! If you try the lighter tampons but still can't fit anything or want to use bigger ones I would suggest visiting a gynecologist. If you experience really bad cramping and clotting around and during your period then it could actually be caused by this! For most women the hymen naturally becomes perforated and bits fall out during your period or even during exercise, but it's possible it might need a little bit of help.