But I can reread the same piece of writing I though was great and a week later wonder what the hell I was thinking.
Most writers I know feel the same way. So stop reading your writing a week later.
What do you mean by being able to tell how good you are? Good in what sense? Good at writing stand out sentences? Good at drawing lifelike characters? Good at dialogue, or mood and tone, or pace and flow? Good at telling whether you know what a story really is? Good at writing the kind of stories editors want?
I don't even try to judge such things because I'm wrong as often as I'm right. That said, some writers I I respect greatly say they do know when they write their first great, special, publishable story. Maybe I don't try because my first story was like that. I don't know. But I just don't see the point in trying to tell how good I am. Really, unless you're looking for a reason to quit, what difference does it make what you think of your own writing?
Writers, and beta readers, worry incessantly about how a story is told, which matters very little. It's what the story has to say that makes it publishable. The writing needs to be reasonably good, but nowhere near stellar, as long as the story has something to say, as long as it tells a truth people want to hear.
When Ray Bradbury finally wrote "The Lake", he said, I finallyfound it one afternoon when I was twenty-two yearsold. I wrote the title "The Lake" on thefirst page of a story thatfinisheditself two hours later. Two hours after that I was sitting at my typewriter outon a porch in the sun, with tears running offthe tip ofmy nose, and the hair on my neck standing up.Why the arousalof hair and the dripping nose? I realizedI had at last written a really fine story. The first, in ten yearsof writing.
The Lake was no better written than the story he wrote the week before, or six months before, or two years before. It was great because he knew he'd finally told the right story, and that said the right thing.
You have to learn to do the same, and I seriously don't think anyone can help you do this. Certainly not beta readers. You have to read everything, write much, and learn which story to tell. The right stories are always in you, which is why no one else can help. The Lake was inside Ray Bradbury, a part of him, and while the events were fictional, what the story had to say, the truth of it, came directly from his own life experience.
Stop worrying about your success rate, about how often those around you sell, and about the "quality" of your writing. You can certainly write well enough to sell, but you still need to know what a story really is, what editors really want, and how to get yourself into a story, how to tell the flat out truth in a way people want to hear.