I am guessing one of two things:

Performance anxiety is probably common. I can't agree with "perfectly normal".
I'm well into middle age and when I can't perform it's usually because I am disappointed. Either my expectations were too high, or there was a blatant bait and switch. The second shot on goal is where my age or sub-optimal health really shows.
Collectively our hormone health is not good. It probably starts with insulin followed by cortisol.

They've seen it all.  

and still get paid!

It's more work, because they  keep blowing me till eventually I get hard enough to cum. Sometimes that uses up most of the hour.

No, I've never once thought that it was due to a lack of attraction.

Debra's answer is exactly why I think so highly of SW's.  They are some of the best people I've ever known!

This is a sugar baby's dream.  Actually it's an old dude that she doesn't have to fuck. Middle aged probably won't be dead soon enough and are too sharp to change their estate plan.

to be the easiest money I ever made.  

"how can I help him enjoy his time the most?"  Then the answer is to hand him his money back at the end.  LOL

I think nothing of it. I’ve had regulars that never got it up. And honestly I never considered the possibility of them not being attracted to me. It’s actually pretty common. I like to think there is a little more to our experience together than that.  

To OP

I once wrote a blog about this. The title was Vi*g* (not sure if I can write that) is just reading glasses for your dick. Once middle age comes, for many guys getting it hard and keeping it hard gets more difficult. It is something to discuss with your doctor because if it isn't mental they can prescribe something. Just like the bulk of people over 45 seem to need reading glasses, men may need the little pill. There is no shame to this.
 
  But I also always say the biggest sex organ is your brain and once your dick doesn't do what you want your brain gets all involved making it even more difficult. There are so many things to do that don't require your dick to be hard so relax. If you have fun then it's money well spent. Don't go into like I do w buffets thinking I got to eat everything when I should just eat little bits of this and that equal to a regular meal and be happy. Setting yourself up to do this and that or you won't get your money's worth is bound to effect the outcome.

Better luck next time
Xo

Many clients are happy to pay for a job well done.  

It means you are no longer paying her to "go away" and she has EVERY right to treat you like a boyfriend (both good and bad parts)  who's role is to make HER happy rather than a paying customer who has/had the right to make it all about you.  

many ladies tell me that some level of ED is very common.   even studly guys being the first time with a specific lady, or too nervous or too many drinks before LOl ..    so the best ladies attitude is, i want to make the gentleman happy,  both to get nice review as well as repeat customer and just understanding that many guys are simply seeking a positive intimate experience.  

Honestly I have never thought it was because of not being attracted to me.  

A man can get excited seeing a sexy M&M. I wouldn't assume it was because they weren't attracted to me and frankly men are quite brutally honest.  Most of the men I know would just leave if a provider wasn't what they wanted or expected. I assume its nerves and offer a massage that always works and takes away the pressure.

Here's something no one mentioned so far: medications.  One example I know off the top of my head is antidepressants, SSRI or SNRI class in particular.  Those are notorious for causing erectile issues and/or difficulty orgasming.  Dopamine is responsible for sexual pleasure and its orgasmic effects, while serotonin is responsible for that mood boost after sex.  But when SSRI's or SNRI's raise levels of serotonin, it crowds out the dopamine, because the number of hormonal receptors in the brain is limited.  On the plus side, it makes sex a more "heady", psychological experience.  In layman's terms, it means  a person enjoys it more on an emotional level, despite feeling less animalistic pleasure.  

Definitely more work!

in my experience, hooker-girlfriends seem to have a sixth sense for knowing when its time to go away so they don't become clingy or annoying.  Some of the best FWB's I've had with hookers have been on again and off again rather than continuously. You have a couple of intense weeks during a "reunion" then you may not see them for two or three months, then it repeats.