As a "Solopreneur" you must understand a few things;
1) You're money is limited so spend it on the 20% of the tasks that will deliver 80% of the results (Pareto the MF)
2) Your Time (unfortunately) = Money (for most people) , which means that you are investing money when you're working on your project.
3) You are your investor - having a side job removes the pressure of "needing to sell" which is desperation and desperation is like dousing your salad with bull semen - nobody wants it.
4) Take care of your basic needs (non-negotiables include all basic necessities). If this isn't being met, then you must find some way to ensure this is taken care of because this spirals out of control quickly. If a part time gig can cover it, that's more than enough.
5) Be Water - things will break, not work, fail miserably. That's okay - adapt, change, pivot. This is the fundamental key to surviving all of it, don't be married to "one way" of doing things. Maybe your ways are outdated - scrap them.
one phrase that became a mantra for me;
"Winner Quit Often and when they do, they quit fast and without remorse".
In other words, if it ain't working to benefit your bottom line, cut it out and don't even think twice about it.
There's no time to chase vanity. If you have a real solution, believe in your vision, do the diligence - there still is no guarantee that the market will adopt it. But with the right marketing, branding, and strategic execution - you can make market adoption be more fluid and organic, less effort, and simply "fucking forward" as they say (whoever they are).
Anyhow, I know it's hard, but hopefully there's something in these words that can be a nugget of inspiration.
Take a breath, relax - think clearly. You got this!