One of the big issues I'm seeing more and more is we tend to put a value on our work based on a number given to us by a website that has it's own motives rather than turning to people we trust (like teachers, family, friends, or clients) for their input. The problem with that is each website has a brand, and the people on that website are ONLY looking for that brand. So instead of seeing these platforms as a proof of your validity, see it instead as a publisher that has it's own focus and it's own agenda.
If your project gets rejected, it's the same as sending a project into a publisher and getting that rejection letter. It happens. It happens to all of us. And always remember that Tapas could be gone in 10 years. Think back to Myspace, think back to Neopets, think back to LJ--these sites come and go. The art you make is forever.
So to answer your questions, because I've had so many years of feeling similar feelings (my comic has just one less sub than yours so we're in the same pool 
)
How to keep believing in yourself when there's hardly anyone else who does?
Summary
I think it's important to remember the value of a human being. The numbers artists deal with nowadays are in the expectations of 10,000's. That is like...the size of a small city. At no other point in history has an individual visual artist had to drum up that type of following all alone outside of like...modern art and theater. So, don't stress out if it hasn't happened yet. It's extremely unnatural to do and is not normal. This doesn't just happen--it takes so much time and targeted marketing.
From watching others, I've found that having higher numbers does not mean that those artists are magically cured of their insecurities. If you have insecurities now...it will still happen when you go viral. Now I'm not saying that it's bad to be viral--it would help us out a great deal if it was easier to make a dollar OMG, we would all benefit from that. But, those insecurities will still be there. That's just how insecurities work. They never really go away.
So I try to remind myself, when I'm feeling bad about numbers, that in terms of making money and getting hired, you're going to find yourself in an interview one day, and that interview will be with one person. Just one. That will be the only person who matters. If you're interested in going premium on tapas, if that is your goal, then guess what? You have a slightly larger team of like...10 people who will matter. If you're sending something to an editor at a magazine or a brick and mortar-- then that's the only person that matters.
It's a lot less overwhelming to work on a project when you already know WHO you're going to pitch it to, and to make sure that it's not a number of like...half the planet, youknow? I can't make 10,000+ people happy. I can't comprehend that many people. But, can I make a single client happy? Yes, I can. I can do this.
Now I still care that I don't have a ton of followers, but at the same time, I'm well aware that the only numbers that matter are the ones that know me and hire me. When you realize that, you make those followers you have feel a lot more valued. One might buy your stuff. One might have really good input. You really just never know who's watching.
I remember I made a patreon back in the day (inactive now, I don't have the time) and I figured no one would join--but immediately I got a donor, and it wasn't from my social media sites with 700+ followers, it was from my twitch account with 15 followers where I only streamed art once a week. You just never know where the people who actually care will come from.
How to like your art when nobody knows or cares it exists?
Summary
When it comes to my art, I'm making my own comic not because of it's views (it has very few) but I'm making this comic because I want to read my comic just the way it is. I'm excited to make this weird ass thing real, and making it is this fun experimental sandbox feeling because no one really reads it, so I can do whatever the hell I want. I'm making it because no one else is, and if I found someone else making a comic just like mine, I'd slam that delete button and just read theirs so I don't have to do all of this work.
But another reason is because I know the works I make after this will be that much better. It's easier with paintings and illustrations to just move on from a project when it doesn't pan out--because you can make a new painting every day. I think that mentality helps me a great deal when it comes to comics. I'm used to making things no one sees, because 1/1000 will get seen, and will get viral. But, you just have to keep trying new things, keep learning things, keep on swimming when you make duds. Everyone makes mostly duds, but are only known for their popular works.
How to be proud of your improvement even though there isn't an increase in your popularity to go with it?
Summary
So when I was starting art I was very bad compared to everyone around me because I didn't get serious until college. Something I learned was that I needed to focus on what I did well in every painting I did in order to not get blindsided by the competition.
If you are making pieces just for online numbers, that does not mean you're making improvements. Works we do to improve are rarely that interesting to get views and followers, that's just kind of how it is. Doing studies of hands, or paint strokes, or getting better at comics even, is a long and kind of boring looking process. It's important--you have to do it--but popularity posts are different from improvement posts.
Like I think my most popular post was getting a retweet of a painting I made of DiPiglio from Chris Fleming. I was very thankful and am eternally grateful to him. But youknow what? It was one of my worst works. Sure it got viral but like...wish he did something I actually put time and effort into, youknow? But eh, that's how viral stuff works!
How to avoid finding yourself worthless as an artist because you know you will never make it?
Summary
Something you have to avoid when it comes to anxiety or depression or any of the big scary mental illnesses that artists have to deal with because we have to put ourselves out there all the time, is to avoid all-encompassing phrases that are absolutes. Saying something like "worthless" or "never" is one of those phrases where if you catch yourself thinking it, you have to talk back to yourself. I have insomnia problems so I have to do this in the day, because at night-time I am too tired to talk back. So I memorize answers to say to myself when I am sleepy and feeling futile.
So how to you talk back to "never?" For me, it's because "never" is statistically impossible. Me, I'm in my 30's. I have a lot of life left to live. I have a lot more stories to write and to improve with. To say "never" is a fallacy. That is just my borked brain talking.
And then to talk back to "worthless" I remind myself that art is just a job. You're gonna have a lot of jobs. I had really bad carpal tunnel that made it so I couldn't do art for a year. I had to redefine myself not as an "artist" but as...me. That was hard, but it was really good for me to see that...there's a LOT of life outside of art. It's just one facet of what we do in a very niche community. You do so much that has worth.
Another thing to help with imposter syndrome, is to write down every single nice comment you get about your work. Keep that somewhere where you can look at it, and you'll see that there's...so many things we forget in our times when we're feeling down.
