Has happened to me before, and I suppose it will happen to me again. It was never an issue of size, but always some kind of unharmonious, emotional vibe. It is an intimate act, and I believe sometimes we want something and don't want it at the same time -- there are fears and stigmas attached to mostly everything sexual, and most of us have to work through them at some point or another. To me, it was not so much a problem when I was still vanilla (no big deal then), but when I began to explore the submissive side of me...let me tell you, I often struggled, and lots of it. With me, with him -- and I needed him to 'force' me because that made it okay. It wasn't me driving the action, and as he was driving the action he could not think poorly about me after or during play.

There are new experiences to be made here. It's experiences that are wonderful and scary at the same time, feelings that are very intense and which can span such a range of emotion. It is no wonder that you are nervous, and it is no wonder that your nervosity makes you tighten up. It has happened to others before, and others before you pulled through it. But most of all -- it is perfectly okay to feel that way. You are not disappointing anyone (but yourself, and you don't need to be). Sexuality is so much more than only the act of penetration, and if it takes time to get there (and, perhaps, getting to know your partner very well, to establish a bond of trust -- which I don't doubt you have, but I understand that you don't know each other all that long yet?) it takes time to get there. I know the feeling of not being able to 'perform' like you think you should perform is horrible, particularly if you actually WANT to do the thing in question. Yet, sometimes things don't work like we want them to work -- and that's okay. Knowing and believing that whatever happens, happens -- that whatever happens is okay usually helps me relax. Maybe it will help you too.

Second thing that really helps is arousal. Lots of it. Lots and lots and lots of it. That's why I'm not sure if trying it the vanilla way is the best way to go -- at least not if you feel aroused and comfortable in your fantasy scene. I mean, you should go the way you feel most comfortable with, and as others said before take away some pressure by not making the intercourse the explicit goal of the session. If it happens, great. If it doesn't -- well, you sure had some fun nonetheless. And that's what it is all about...feeling good, enjoying oneself and the other, having fun.

I too believe that practicing a little might help. If you enjoy yourself and become more used to the feeling of being penetrated -- and, more importantly, have some fond memories of it...well, if you know what you are dealing with, and if you know that it's pleasurable and not painful under the right circumstances -- nothing to be nervous about anymore, right?

Anyway, I wish you all the best and hope that everything will work out the way you want to really soon! I hope I was a little bit helpful.