Been there, done that. I screwed up my first solo checkout three or four times! I thought I'd never do it. I was so, so, so depressed about it all. Eventually, my lovely regular instructor upgraded, and he could send me solo.
Now, I can look at it from the other side. I have a student who I think is pretty close to first solo. But I'm a newish instructor; I've never had anyone ready for a first solo before; it's a first for me. Will I know for certain if he's ready? Nope...which is why he'll fly with a more experienced instructor first, phew!!! But if that instructor won't send him solo, who will he blame? Himself, of course; we all do - though it could well be me who got it wrong. I'm already planning to try and get round it by just telling him I want him to fly with the CFI to make sure all's going OK, to teach him a bit more about emergencies in the circuit, or something like that anyway. But he's not stupid; he'll probably guess.
So you see, modelman, there's quite a lot going on here. Possibly you're not quite ready, possibly you are but were naturally nervous with another instructor. Either way, try not to get too demoralised. Keep flying, keep learning, and someday soon you'll get to do exactly the same as you're doing now, but with no-one next to you. That's all it is really. It's not that big a deal...but of course it feels like it.