My answer is a little broad and maybe even opinion-based… so here goes.
I think you can divide your decision process into 3 "problem areas" – it's a little difficult to say these things in a neutral nonjudgemental way, so hopefully you will have the patience to translate my words into ideas. I say the word "problem" but the reality is there may not be a problem at all, just that the odds are stacked against an unestablished author.
- there is a problem with the book
- there is a problem with the publishing industry
- there is a problem with the author
1. There is a problem with the book.
(See GGx's answer about an "unmarketable" book.)
You haven't mentioned beta-readers, or whether you have gotten any feedback on the story. I'll assume you've asked family and friends to read it. They may not be able to give you critical feedback because they cannot separate the book from the author. They care about the book because they care about you, so they may not be able to approach the book is its own thing. The goal is to get "honest" feedback from strangers, strangers you do not need to please and who do not know you personally. You'll need a range of beta-readers, not just genre fans but include them too.
The next step up might be to hire a professional book doctor, sometimes called a book doula (the person who assists with a natural birth). This is a professional writer, with references, who for a fee will work with you to "fix" the book. You'd want to be sure the problem is the book though before throwing money at someone to "fix" it.
2. There is a problem with the publishing industry.
Possibly the problem is your approach (see the other fine answers here), or lack of access to the publishing industry. Statistically, not many books are "discovered" through cold submissions to publishers and agents, probably less chance than winning the lottery (*not an actual statistic). I tend to roll my eyes everytime I read so-and-so's incredible first novel got a huge cash advance, and then reading their bio it turns out they are an editor or similar at the publishing house. Or they are a TV producer, or otherwise employed in production, promotions, or publishing.
Recently, a pseudonymous Italian author called Elena Ferrante, praised for her "fascinating novels about the lives of women", was outed as a former publishing house employee. Let me put on a surprised face that this "wondrous" unknown author who "took the literary world by storm" was an insider. When does this get called a hoax? Or a scam?
This has nothing to do with your book so you can't take it personally. Projects tend to be promoted in-house, and anyone working within the publishing industry has the personal and professional connections to "jump the line" over random submissions from an unknown. We can't even be sure your book has ever been read by a publisher. If there is no concrete feedback about your work specifically, I think you should assume they are not even opening the envelope (that may not be true, but without evidence to show otherwise the generic returns suggest it).
How can this "problem" be solved? Get a job in the industry and make personal and professional connections for yourself, or try self-publishing. Both have learning curves and time commitments (probably expenses too) that take you away from being an author. There are technical considerations, which might not be your goal, and it's not a guarantee that you will actually reach an audience. Either way, it's a hustle to promote and manage. You can easily go with a print-on-demand service, like Amazon. I'm actually surprised that so many writers here seem to reject self-publishing as a non-starter, but self-publishing your own fiction isn't the same as self-publishing non-fiction. More than half the money spent by the entertainment industry is on advertising and promotion. No individual can self-promote on that scale. You are competing against every other form of entertainment available.
How important is it to you that the book itself be "finalized" into its corporeal form (ie: a paper book). Can you be satisfied with ePub and print-on-demand? Would you be happier to see the book abandoned and forgotten if it isn't published through a third-party?
3. There is a problem with the author.
You just aren't famous enough, and there are hundreds of already-published authors with mediocre 2nd books ready to go. Even worse, there are tens of thousands of celebrities whose name could be slapped on a book cover. The actress Adrienne Barbeau has a fiction series about a Los Angeles cop dating a vampire (the first was co-written by a professional author). I'm not trying to insult her, good for her, but how many "cop dating a vampire" stories are written by unknowns every year? It doesn't sound very original. I'd say her name-recognition went a long way.
Why should publishers take a risk on an unknown, even if your story is better? Authors need publicity and fans. If a reviewer can mention your earlier novels, and how this current one compares, they have a built-in frame story to talk about your latest opus. If there is no backstory to talk about, they can really only talk about this novel. From a reviewer/journalist/blogger's point-of-view inventing a compelling story about a 1st-time author is a lot harder than talking about your past books and then segueing into the latest book.
And again, the publishing industry really seems like a scam when authors are praised for fake memoirs, and their publisher backs them up claiming the stories were all vetted and true (until they are exposed). No problem because now he's famous and can get his terrible screenplays bought and made into movies. Remember when I mentioned some "instant success" authors work in television? One hand washes the other. Their entire industry is about making talentless nobodies into an overnight success, why shouldn't they be the talentless nobodies getting famous and signing contracts? Who are you again? What is your highly-marketable backstory? Were you homeless and living in a car while scribbling YA wizard stories on a napkin? Was your first novel written in rehab (or did you claim it was)?
Publishing, like Reality TV, is an unregulated scam world filled with P.T. Barnum's selling flim-flam stories. When they get caught, the publicity only helps them. If you aren't a narcissistic sociopath, I'm not sure how you can hope to compete in their arena.
4. There is no problem, this is how the world works.
Well.... Now what? Live your life? Write another book? I don't think anyone can tell you what to do.
Enjoy your time on Earth and don't judge yourself or your creative work against unrealistic goalposts established by literary hoaxes designed to be free advertising. My personal advice is to get a little distance, grieve the end of your relationship (or celebrate the birth, whatever metaphor you choose). Attend to your mental health and re-center your creativity. What do you want to do?
You've finished a novel! You are already a winner. Getting "discovered" is another goal altogether.