I've been in the writing game for a long time (specifically, I've written ten novels over the course of ten years) and, to be frank, I've had hardly any success out of hundreds of queries sent to literary agents (basically, only a few minuscule nibbles and a solitary full manuscript request, ending in a form rejection). At this point in my career I can't help but be confused, since I have an MFA in Writing Popular Fiction, have been meticulously mentored by small a press editor and several authors of big six publishers, and yet my success rate has been truly abysmal.
Over the years I've also heard many agents and editors say that based on my type of track record, that means my writing is just not at a high-enough level to gain the interest of agents. However, every once in a while I also hear about success stories in which an unagented writer who claimed to have almost zero agent interest in a novel submitted that novel during a large publisher's open-submission period and - viola - they not only got a deal but their book sold well.
So this makes me pause. Is it sometimes possible (perhaps albeit not so frequently) to actually be writing at a very high, publishable level, but still be hardly getting any agent requests or interest over the course of a few years and countless queries? Or would you say the writer is still not up to snuff and may not even be talented enough to make it in this insane, shark-infested business?
It's entirely possible to be a talented writer and not get any interest in the books you write.
In addition to talent, you need skill. Generally you'll build your skills with practice, but practice without coaching isn't useful.
The coaching you're getting though (the MFA, the help from other writers etc) might not be doing the job. I'd suggest you get some from the people you're querying: agents.
If you have some cash to invest, you might consider one of those "I'll critique your novel for a charitable donation" kind of thing that crop up periodically.
The other reason that well-written novels fail to catch an agent's interest is they're not books we want to read.
I see well-written books that are non-starters all the time. There are a couple reasons for this: the novel feels like something out a 70's TV show; the characters are people I wouldn't want to ride the subway with, let alone pay $25 to invite into my lair; the topic bores the sox off me.
On the other hand.
It's entirely possible that you're writing something agents don't want to read but other people might. To that end, open submissions at publishers are a great idea. Another good one is posting to platforms that get your book in front of readers (Wattpad etc.)
I have a client who is prolific and published. He thinks about one in eight of his novels are publishable. I'm not sure that's accurate but I do know I've read more of his novels than I've sold.
That means you need to pick which novel you think is your best work for this critique/open submission/Wattpad. If you don't know or can't decide you'd probably do well to choose the most recent one, given I hope you're improving with each new book.
And my best tip on how to assess your own work is to write out a novel you love.
If you love Jack Reacher novels, pick your favorite. You'll inhale his rhythm and syntax without thinking about it. You'll see where he turns a reader's expectations upside down. You'll see where he surprises his readers. (If you're not surprising your readers, you're going to start boring them.)
This is a really difficult problem and you should expect to spend some time and effort analyzing it. There's no quick solution.