I had this problem for years before I lost my virginity. For me, my cherry just had to be popped, and then anything smaller was fine. I still have a problem with anything larger than what I had my first time, but that's just because I'm naturally tight because I'm a small person. You might be the same way.
Your first time having sex will probably hurt more than you can imagine
So I also recomend seeing a doctor before that happens. Maybe there's something he can do to help loosen up, before you have to go through that pain?
-Melissa-