You can't get there.

You pound fast and hard as she ripples from head to toe, rocked by a nice orgasm. Her pussy is contracting on your cock as you thrust, and it feels amazing.
But you're plagued by the old familiar feeling. It's not getting there. The sensations aren't quite right, in the right places. Fatigue is starting to creep in, and overstimulation is fighting down the necessary pleasant stimulus that would bring you to your ultimate goal.
You roll Lucy on her side, hoping to change up the angle and sensations. "Fuck....baby...I didn't know you were a stallion. You've got staying power..."
You ignore the compliment and try to concentrate on your pleasure. You vary your speed, and angle, and fondle her tits and smack her ass, all in an effort to kickstart your orgasm. After a few minutes, you resign yourself. Eventually, your erection will give out. You slow, and pull out, looking at your softening cock with anger as you lay back.
"What's wrong, baby?" Lucy looks at you with concern.
"It's nothing. I'm just getting old. I jerk it too much. Can't always get there." You don't like the look of sympathy on her face.
"Should I help you...?" She grabs your cock in her hands, but you put yours on top of hers.
"No, it's fine. It's not going to happen. I have to give it time to relax. If I had more time, we could go again, but I can't stick around all day."
"Oh... ok...." she looks... disappointed? She got hers, what's the problem?
"It's not you. It happens with my wife, too." Why are you trying to comfort the woman who enticed you into a tryst? You guess it's just second nature to explain away your problem.
"Maybe...some other time..."
"Maybe..." now, why did you go and say that? This can't become a regular thing. "I'm going to..." you indicate toward the laundry room.
"Yeah... probably a good idea."
You get dressed in shorts that are still slightly damp. You give Lucy a little kiss before you go. "Thanks. Like I said...it's not your fault."
"I kind of....you crossed the line...broke your "vows" and i couldn't get you a nut for your trouble."
"Don't worry about it." You take your leave, heading back to the clubhouse and climbing into your car, trying to forget the look of pity in her eyes. You head home, rolling into the driveway around 2:20 pm.