All I can really do is agree with others, and lament that there aren't many safe places to discuss this side of the business. Just know that you aren't alone. And that I would love to have 5000 GR ratings.

With that many readers, you must have some fans, no? Does it help at all to hear from readers who loved your first book? If you're like me, you tend to overvalue the negative voices and undervalue the positive ones, so it's important to be aware of that and counteract it. Have faith in the readers who loved your book, and that could be a step to regaining faith in yourself.

And try to stop hearing your current agent's voice in your head. There are ways to tell an author, "No, your book did not sell as well as we hoped" without calling it a failure. Granted, not all agents are going to be tactful, but I think being supportive is part of the job. If the agent no longer feels able to be supportive of your career, the relationship isn't working.

I agree with RaggyCat that showing work to CPs and betas helps. Best of all if you can work out an in-person group with people who are also published and writing in similar genres. I lucked into such a situation, despite living in the middle of nowhere, and it's been hugely, hugely helpful to me. I now have a buffer of sympathetic-but-critical readers against the scary, high-stakes world of publishing.

Also, before I was a published writer, I was a book reviewer keeping tabs on local authors' careers, and I've seen situations like this so many times. A great debut without a follow-up, a few wonderful books without a follow-up, a long space between books. I've heard so many authors' complaints about their agents and publishers and publicists. I've seen successful authors of an earlier era turning to self-publishing because they couldn't get a trade deal. I've seen wild comeback stories, too. The "instant success" stories you see on social media are just the stories people are eager to tell.