Calm down - you're stressing out and worried, and therefore your muscles are tensing up too much, that's all it is. I promise you, once you're properly relaxed about it, it'll be fine. Just remember that if a baby's head can get out of there, a tampon can certainly get in!
It might help for you, maybe in a bath or somewhere relaxed, to try putting a finger in and just proving to yourself what you can and cannot do. If you can't get a finer in, you're definitely too tense, and if so you might want to have a look at vaginismus - and if it is that, most women deal with it at some point so it's not that big a deal and easily sorted out. All you need to do is try and relax and stop stressing yourself out by telling yourself that every time you try it'll be really painful and you won't be able to have sex, because that isn't true. Ok?
Good luck with it.