It can happen if the chemistry with the provider isn't quite right or if you've had a crappy day before your appt.  Been there done that...

I get nervous too.  I think it's a combination of having no sex whatsoever in the home life, the anticipation and buildup of the provider date, and one's general characteristics (worry-wart type of person).  Things have gotten better with the more gals I've seen, and especially if I see the same gal more than once.  

I wonder in your case how much of the difference the 2nd time around was due to the "magic pills" and how much was due to just being your 2nd time with the same gal.

The other two replies are very insightful.  Chemistry, the kind of day you've had, your home life all matter.  Quite frankly, you don't seem like the type who'd see a lady as one dimensional.

Positive self talk helps.  Visualize yourself connecting with the lady you choose to be with, and satisfying her.  Do this until a happy outcome (for both) seems very real.  Chances are it carries over into the actual meeting.

Be comfortable in your own skin.  For both sexes, it is true that each of us has unique things to offer the other sex.  Draw confidence from your strengths.  Women are a lot better than us at picking up on this self awareness.

This happens all the time with newbies with me. Men are unfortunate in two ways....1)they can only usually climax once and then need a break. 2)drunkeness,stage fright,nervousess,not attraction etc. can make the man not be able to perform. Women can do it anytime

If nothing happens...and I mean no penetration,I usually offer at least half back.That's only fair! That way they see me as fair and nice and maybe they will try with me again. After all let's be frank...he did not give me my fee for a nice conversation! If I were a legitimate business you have to refund money if the service you promise is not provided.