STRESSSSSSSSSSSSSSSSSSSSSS!!!!!!!!!!!!!!!!!!!
That is the problem, good and bad stress can stop a finish. Stress and medications, but I would bet on stress. Cathy is right, you need to find a SP who can tell when you are tense. I try to never start a session with a man until he is completely relaxed and most times that means giving a full massage with some small talk. Just help relax the mood a bit Also, watching the clock is not good for you either, whether it is her or you watching it. Try booking with an Independent SP, they are more likely to be able to go a little over time with you if it is needed.
One more suggestion, talk to your SP, and guys, this goes for all of you. We can only give you service that you express you like. Yes, there are the norms of DATY, BJ, MSOG, etc but there is more to the SP experience then just the norms. If there are areas of difficulty, don't be embarrassed, let us know. This is our job. We know (most of us) what we are doing. You are paying anywhere from 150-400 dollars, we better know what we are doing. There are so many different aspects to sex and the sexual experiences that you have, but like most things in life, communication is the key.
For example, attire can play a big role. Most older gentlemen love the feeling of nylons on a woman during sex. I know one gentleman who cannot finish if a SP is not wearing them. It is what he is use to.
Hopefully we will soon see a lovely review of the special SP, who was able to help you out, so be sure to let us know. If the problem becomes persistent, go to your doctor.