I want to add to thins that the idea that the vagina or vaginal opening is something that can be "stretched" or made larger in any permanent way is myth, not fact.
In reality, what happens is this: that opening and canal are actually pretty elastic, and how open they are is just about what, if anything, is inside them. When nothing is, the vagina is a closed tube: it doesn't just sit there open. In ideal conditions, when something is inside of it, that is when it stretches. When that something is removed, it's basically: boing! -- it goes back to being closed again just like it was before.
What makes that anatomy more flexible than not is arousal -- being very turned on -- adequate lubrication (which often will mean using lube) and emotional comfort. If you figure you will be in pain, for the record, you're going to psych yourself out and probably WILL experience pain (and less vaginal flexibility) when you might not have otherwise. A larger penis does not, by any means, mean someone having intercourse with the person with that penis will experience pain just because of size. On the whole, penis size usually has very little to do with pain with intercourse, while things like not being turned on, anticipating pain, not communicating, not using enough lube, and partners being hasty have a lot to do with it.
It sounds like, for you, you're just going to need some more time to get comfortable -- and being more comfortable has a lot to do with things feeling good instead of painful -- with maybe even the idea of something in your vagina, whether that's a tampon, a finger, or a penis. Take that gradually, going with your own pace: give that all the time you need. There's no need to rush it, especially because if a finger or two doesn't feel good -- or okay for you -- a penis isn't likely to, either. And the point of all this is not about not being in pain, it's supposed to be about feeling *good*.
