Okay okay, deep breaths, buddy, you're not screwed. You've had a little stumble and it's thrown you off-balance. I've seen comic artists come back from way worse than this. Your situation is really not all that bad. Objectively the problem is:
You have made a comic which has excellent art and solid gag writing, which has 13,000 bookmarks (incredible, that's a bigger number of bookmarks than the average per-issue readership on most published comics!), but has hit a point where it can't expand further easily and fails to generate the kind of buzz you need to really push your career to the next level because it has already filled its small niche and isn't a wider appeal kind of comic.
You are a hermit crab who has survived long enough and been successful enough to outgrow its shell. You have made a comic that does what it does very well, it's just that the type of comic you've made would have been a hit in the 00s, but the audience for webcomics is different now.
This is not a horrible situation! What, did you just spend $10,000 on a huge print run of five thousand books? Did you flounce out of your day job and tell every employer in the country you'll never work for them because you're a hot-shot comic artist? Did you stinkbomb the Onipress offices? If no, then there's really no barrier here.
Your options:
- Start a new comic. Yeah, you can. It worked for John Allison, who moved from Bobbins to Scary-Go-Round to Bad Machinery and Giant Days, it worked for David Willis going from It's Walky! to Shortpacked to Dumbing of Age, so there's no reason you can't make a new comic that's more in line with the tastes of the contemporary audience without the baggage of your long page count, and even bring along some of your fave characters from the comic you have and put them in a new context. You'll probably get faster growth from fans moving over from your old comic to help visibility, so you should build up visibility quickly.
- Dig in with the comic you have and merchandise the crap out of your audience. Like, dude, your comic is like an early 00s sexy gag comic, do you know what your core audience is? Men in their 30s and 40s, the biggest spenders in comics merch. I've seen it bandied around that if you have a core base of 2000 really hardcore fans, you can scrape a living off a webcomic. You could sell posters, books....body pillows....
(I'm not necessarily saying I'd approve, but as a business decision it makes complete sense given the genre of the comic).
- Start pitching to publishers and showing your portfolio at events. Your work is more than good enough, why not give Dark Horse, Oni Press or even Marvel a shot?
- Have your comic as a thing you do for fun on the side and do a different job to pay rent. There's zero shame in this and it doesn't make you a failure. Being pro full time for like... nearly everyone I know on the scene means drawing crappy comics about other people's IP. A lot of my friends who wanted to draw the comic they care about started doing a different job and having their dream comic be a side hustle or even a hobby (self included, actually).
You have plenty of options, and whether you drop it or continue, you have a very successful webcomic under your belt that you should treat like an asset, not a failure. You could go to any publisher and say "I have kept regular updates for five years on a webcomic with over 13,000 readers, and now I want to take my career to the next level" and they'd be really impressed.