First, even in developed countries, as where I am, the job market is very competitive. And that is even harder if you don't have a degree, or years of experience and the connections to get another.
With that said...in my opinion, there are two different scenarios.
Scenario 1 - full time job, not much income coming in and just starting
Scenario 2 - full time job + a full time freelance business or other.
1 - Don't quit until you have 3-6 months of consistent hours that will sustain your lifestyle, plus 3+ months of living expenses, including business expenses.
2 - Your health is #1. If the full time + freelance is interfering with that, start planning an exit plan. If you get sick, where are you then? Do you get paid by your job? Can you continue freelancing? It's not worth it and WILL set you back in the end.
It sounds like you have consistent work. Does it sustain you? If so, make your plan...maybe 2 weeks or 1 month of saving as much as you can, and then resigning.
Fear of the freelance drying up, will keep you where you are at. Fear is what stops people from taking some very worthwhile risk.
You sound like you're in a great position to freelance, and you will have people that recommend you. You will have the energy to market yourself. You will have time to work in a more focused fashion and get more done in a smaller amount of time.
So, in reality, you will gain more then your full time hours for marketing, sleep and balance.
Congratulations. Live. And plan as much as you can. Keep a positive mindset and things will fall into place and direct you the way you are to go.