I have a few thoughts.
1. Focus on finances. You say you're behind on your car payments. With the upside-down state of the current car market, there's an outside chance you could sell it at a profit, maybe clear 10k. (I have no idea what kind of car you have, but 10k > sales price is not unusual right now.) Maybe you could sell it, pay off the note, and have enough left to buy a very basic used car?
2. SSDI. A disability law firm can be helpful. I used them for LTD but they exist for SSDI as well. For LTD they work on a commission of your back pay once you are approved. YMMV.
3. Part-time work - at this point your focus needs to be on survival. What can you do, that's relatively in demand? Start there, worry about perfection in your career path another time. P/T flexible jobs for disabled people -- that's a need that's been unmet for years, but there are some developments in it. Try https://www.wearecapable.org// .
4. Keep up with the therapy. Also investigate nutritional deficiencies, thyroid imbalances, etc. Fixing them won't "fix" your problems, but it'll give you more physical and mental strength to deal with them. For more info see my short essay: https://radiosof.com/2019/09/22/some-interesting-tools-for-d.... The embedded links also link to some other essays on genetic and nutritional components to depression. Some of these can also affect inflammatory response as well. So finding out if you're bad at methylation and supplementing with B-12 shots and methylfolate (for example) could also potentially reduce some pain levels somewhat. Which would then allow you to think and strategize more clearly.
5. Depending on your issues, it's great to have really smart, genius-level doctors on your side. The Center for Complex Diseases in Mountain View is one such a place. I'm not sure chronic pain is their specialty, but perhaps you can call them and ask. I think Stanford has a pain clinic as well?
You're used to the YC world, where they talk about "10x" coders. Well, there are "10x" doctors. Once you get into the world of invisible hard-to-treat illnesses and you're trying to qualify for disability, you NEED a 10x doctor. First, b/c they might help you improve. And second, because they see people like you all day and they are going to be better at helping you qualify for disability than a regular primary care doctor.
I would recommend calling the Center for Complex Diseases, and asking if they cover patients like you, and if not, who they recommend. Then put the very expensive appointments on your credit card or borrow money, do what you have to. They are an investment in your ability to qualify for SSDI. A few thousands now may guarantee $18k/ year for the foreseeable future.
6. This website is the Bible for people navigating disability applications: https://howtogeton.wordpress.com/
Good Luck!