You're not stupid, and you're not the first person it has happened to. A few things that will help:
1) REALLY make sure she's happy, relaxed and ready for it. It's quite common for girls to tense up a lot even not entirely consciously, but lots of foreplay and feeling comfortable with the situation will help no end.
2) Lube.
3) It may be that her hymen is intact and that's what's causing this "barrier" (though it really just makes the opening narrower; it's not a total blockage). This will mean that it has to be broken, which will be painful and messy. But needs doing, unfortunately. But it needs to be at her pace.
4) Don't panic, cuddle lots and enjoy yourselves, don't feel that you're doing it all wrong. Nobody gets it perfect first time!