Life with my SB is still amazing!  We have a week long vacation coming up soon.  I'll post highlihgts after.

But, the other SB is fizzling out.  Last few dates have gone worse and worse.  We haven't even spoken in two weeks.  I am texting her in a bit to say we should talk, either on the phone or text.  To cut it off.  I'm confident she knows exactly what I mean.  If she hasn't moved on to someone else or decided this isn't for her anymore.  

This happens, emotions fizzle.  Just not as much fun anymore.  We never planned on this getting serious and she is definitely not the type of girl I could have something more than casual fun with.  Ditzy and hot only go so far for me.  She didn't know what Normandy was, to put it in perspective.  Whomever passed her in High School history must have gotten an amazing blow job from her.  And trust me, they are good.

This is mostly me saying meh, it happens, she was cool.  Now, do I go back and find a second SB or just coast with one?

You already know this, and are describing one of the sorta common ways arrangements end.  I had 3 end last year, all 3 of my regulars!  One moved away which we knew was going to happen, one very kindly told me she was concentrating on school/career/child and would have to end our arrangement.  And tragically, the 3rd one passed away. But I've def had some that just fizzled out!  I spent months since last October browsing/DMing/Texting/M&Gs before I found one excellent beautiful young woman who is now my one and only regular SB.  So I'm sorta in the same boat as you.  Should I keep seeking or coast with the one I have?  

I am still seeking, very actively.  Sorting through tons of scammers and fakes, contacting some who ask for $ in advance,  all the usual wasters of time and energy.  But for me, even though it does get depressing at times, it's worth it, cuz I prefer having multiple SBs, not just one.  And the thing I try to keep telling myself is this:  Seeking still works perfectly, that is, when it works!  You notice a profile, DM her, exchange texts, have one lunch date, then start going BCD.  Easy peasy, all green flags all the way through.  That's how it was with the 3 I lost last year as well as my new one.  And that's how I expect it will be with my next one, whenever I find her!

Sorry it's ending like this. For what it's worth, I will offer you the same advice I used to give my mentees' in my 40+ year professional career about communication:  
- Never send an email when you can send a text,
- Never send a text when you can call,
- Never call when you can video chat (added this one about 8 years ago, lol),  
- Never video chat when you can meet face to face.  

 
If you want to be human, nice (but firm), and help her off the ledge gracefully, follow my advice above and call her to explain.  

 
About a "replacement:"  
I learned at a fairly young age (while in college) that I am not cut out for monogamy. To be fair, I learned this from my first 2 serious relationships when each of them cheated on me, multiple times.  The lesson I leaned was that I SUCK at choosing monogamous partners. So I stopped looking.  

 
When I entered the Sugar Bowl, I knew that the appeal of dating multiple women was irresistible.  Of course, the logistics of this is impacted by time, libido and budget.  My SB rotation over the last 10 years has flexed from 0 to 5, typically with a primary SB (we call each other BF or GF) and at least one secondary who I see as schedule permits.  I really don't mind if I end up occasionally going 20-30 days without an SB date when something ends, or some external factor interferes (like her family/job). In LA, it's rather easy to book a pro, find a Stripper who will do OTC or just find a BNB (Bang and Bail) SB to fill the gaps.  

 
So for me, when I lose an A-List SB, I will naturally start looking for someone new.  That doesn't mean I wasn't emotionally invested in the SB who left, or that I won't miss her. It just means I like variety.  

 
Life is good

 
The Cat

And it's gone well.  With the right woman.  As you said, picking the right one.  I'm half way surprised the main hasn't asked me to be a one girl guy.  With the occasional hall pass when she isn't available for a period of time.

Now. With the wrong woman.....  It's painful to put that much trust and to lose it.  So I feel you there.

I'm just wondering if it's worth the hassle.  I logged into SA and saw a few girls that made me horny.

It's normal that relationships end. They run their natural course. If they become forced and uncomfortable over multiple meets and the magic is gone, it's time to let her down gently and move on. Chemistry is so unique and individual. I'm grateful to have the opportunity to meet so many new girls for NSA intimacy and be able to help them at the same time. As for down time, sometimes we need a rest from the bowl. It can be mentally tiring dealing with the indignities of using Seeking. Netnoy, with regard to the question of coasting with one or starting new with another, I would personally start a new one. None of us are getting any younger and it seems to me we should utilize our time in the most productive manner possible, meaning balls deep in a young snatch and bringing her to shattering orgasms. On the other hand, a short rest can refresh and recharge the batteries. As always, be kind to others and go forth with grace and generosity.  

For me, monogamy is just too emotionally dangerous.  I know myself well enough to be extremely wary of putting all my eggs in one fragile basket, emotionally speaking.  I've done it a few times, most recently back in the pandemic.  I was dating an extremely beautiful tall slim model type girl.  We both felt that for safety from covid we should be exclusive with each other, so that's what we did for about 9 months.  I thought it meant she cared for me, so I began to really care for her.  Until she abruptly ghosted on me.  I was a mess, only learned later that she took off to live with a BF in Texas, with zero warning to me.  I know a lot of SDs say don't catch feelings. It's good advice, well worth considering.  But I can't NOT catch feelings, that is, not unless I'm dating more than one SB at a time!  Then I'm fine, I'm more protected from my own tendency to get too emotionally invested in any one SB.  I've been ghosted by others, I think we all have.  But when I've had other arrangements to fall back on the ghosting doesn't bother me much.  So I try real hard to avoid having just one SB at a time.

Sound advice sweetman. I took the liberty of grafting it onto a Sun Tzu saying...

"Know thyself, know thy SB; in a hundred arrangements, you will never have your heart broken"