If stress is the cause, and it very well may be, then you need to discover the cause of the stress and deal with that first. Remember, sex is one of the best medicines to relieve yourself of stress, but stress is also one of the biggest hindrances to good sex. If the issue is your longevity (damn, I wish I could consistenly last an hour, my wife would love it), and it is causing your fiancee pain, perhaps using a lubricant will help. By the way, does she climax during intercourse?

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I wish all the ladies were pies on the shelf...and I was the baker because I'd eat them all myself.
I wish all the ladies were potholes in the road...and I was a dumptruck because I'd fill them with my load.