I'll start off by adding onto this one:
Not everyone gets there at the same pace
I feel like it might be demotivating for some to just hear advice like "you just gotta make some comics and you'll get there" and then think 'well shoot, I've made like 5 complete comics and been working at this for X years and I'm still not 'there'. I see lots of other people make like 1 or 2 warm up comics and then the next one is a hit! What gives!"
Well, what gives is that not everyone develops at the same pace and as much as it sucks, you might just need more hard work and practice than your peers to get "there". There's no use sulking on this fact, just pull yourself up by the bootstraps and keep working at it because you'll never get there if you stop.
However, that leads into another one...
If your work isn't improving at the rate you would like or picking up traction even after multiple years and projects, you may not be pushing your boundaries hard enough. Don't allow your work to stagnate
This one always boggles the mind, if you see someone putting out a fair quantity of work over a long period of time but just... not improving that much. I feel like the main reason for this is usually the creator is staying in their comfort zone too much and so while there is improvement to be seen... the same recurring issues or weaknesses keep repeating over and over because they refuse to branch out or try something different.
One of the best ways that I've heard this phrased is to "swing above your weight class". Always be tackling poses and angles and objects and backgrounds that are a little above your current skill level and learn from the mistakes you make while drawing those things and try to do a little better next time. Research. Reference, Practice. Expand your knowledge and skill set.
You will absolutely improve faster by "swinging above your weight class" even though it will feel uncomfortable a lot of the time and yield some bad results than if you just try to draw the same thing/way a gajillion times and hope to have a muse-inspired breakthrough.
Make sure that your goals and expectations align with what you're doing
This gets into some of the advice that has already been spouted earlier like "if you want to make this a job, treat it as such" but I'll take a slightly different spin (since this is something that I spew all over the forums all the time anyways)
It's important to self-evaluate why you're creating your comic/novel. Is it just for fun? Are you mostly interested in improving your skills? Do you want to just get a lot of readers and see those engagement numbers go up? Do you want to monetize your work? And if so to what degree, as extra pocket money or as your primary income source?
There are no wrong answers or motivations, but it's really frustrating to see authors that have their wires crossed and like... Have their goals be one thing, but the way that they're approaching their work runs completely contrary to said goals and then they wonder why things aren't working out for them lol.
Like just at a basic level, if you're just making something for fun or primarily to improve your skills... Then don't expect huge reader numbers and you probably shouldn't waste time going back to redo the beginning 50 times because you're just wasting time that could be spent learning how to finish a story.
But on the flip side, if a creator is primarily interested in getting big reader/engagement readers or potentially making a living off of comics... then yeah, just treating it like any other hobbyist and expecting a premium offer to fall in your lap isn't going to work most of the time. You need to work hard on polishing your skill set to a professional level, learn how to promote effectively, and accept that not every story is going to appeal to the market and be a hit and be willing to try something new if your current project isn't working. And if your skills aren't there yet? Part of being a professional is about not whining about why you're not succeeding right now, it's about realizing that fact, getting help/feedback if you need it, and putting in a lot of hard work to reach that level (this goes back to "swing above your weight class to get there faster").
I'll make this it's own short one: anytime that you have to spend time writing a forum post complaining about why X Y or Z isn't working out for you, you have time that could be better spent improving your craft.