C--
Please don't feel that you are alone in this situation, because I think to some degree all males have gone through a performance anxiety stage with this. It's not you, it's not her, it's not the woman you are with--it's very natural. Actually, look at is as a blessing in a way--it physically proves your wife's sex appeal to you. My assumption is that you have thought about this alot--which is usually the reason why things take a downward spiral--the more you think and worry about it, the more likely it "ain't" gonna happen--kinda like Murphy's Law--if something can go wrong, it will.
I know you asked "how do I relax?" but that's really only a question you can answer yourself. One thing that may help is that practice makes perfect--the more exposed you are in lifestyle experiences, it will become much easier to relax. I would very much frown upon a solution to relaxing being drinking--that can actually be worse. If you want assistance, don't feel ashamed to try viagra, cialis, or levitra for your swinging events. There are a great deal of men that use it just as piece of mind (said party included).
It's kind of like the line from Bull Durham---"You just learned lesson one--Don't think--it can only hurt the ballclub."
)
Tim