Back when my ex-wife & I were trying to have kids, it seemed like sex became a chore rather than a pleasure. Pressure to perform must be very common. Everything worked out eventually in our case, & I'm sure the same will happen for you & your wife. Feeling that you'll "let her down" might be the bulk of the problem. I hope you can jettison that feeling & just relax. I know...easier said than done. Good luck, dude.