Creating an anonymous account to respond for obvious reasons as well...
I've been a de facto solopreneur for a couple of years now, and I deal with the same feelings every day. I don't particular like the market I'm in—I really don't think there's any kind of social good that comes out of it (arguably its a social ill), although on an individual basis I do like my customers. I have a partner who's been basically AWOL since taking a couple of new jobs (consecutively—neither of us was full time and she's an attorney in her day job). Its basically on me to handle not just the tech but sales, marketing, design, etc, most of which I'm really not good at (everything but tech, basically.) On top of that, because of the way we're structured and the agreement we negotiated when I started trading time for equity, back when she was more active, she's getting about 30% more of the cash-out from an impending sale.
(side rant: I used to be opposed to vesting schedules on a company that's bootstrapped—no more. If you have one or more partners, you need to be on a vesting schedule. Period. There's a reason those are a thing. As far as that goes, I basically made every mistake you can make in getting the company to this place—the fact that I'm even seeing a prospective payday is almost in spite of the mistakes we've made, not because of anything special we did. Someday I'll be able to tell that story.)
On top of all that I rolled out the new version of our platform a month ago and overall its not going great. It was premature but because its been delayed several times and our work is seasonal, there was a hard deadline on getting it out and I had to release it missing several features that were part of the old product. (As I'm writing this, I'm cringing—another rookie mistake from someone who ought to know better.)
All that to say, you're not alone in finding yourself in this position. Something I spend a lot of time thinking about when I'm not fantasizing about accidentally stepping in front of a bus is what gives me meaning and self-worth, and how fucked up it is that so much of that is tied to my career. I'm also a father (so no, won't be intentionally accidentally stepping in front of any busses) and a husband, and when I'm being objective, a friend to many and mentor to more than a few. There's more to me than just my career, but damned if I could prove it right now.
So now I'm really trying to consider how to claw my way out of feeling this way. I see both a therapist (two, actually—one personal and one couples) and a psychiatrist, I'm on an antidepressant, and before anyone goes dropping medical advice without a license— I've got years of experience with different meds so yes, I'm certain this one is the right one for me. No, I'm not exercising or getting enough sleep, and yes, I recognize that's likely part of the problem but its difficult mental hurdle to get over to take time away from (unproductively) coding. As I'm writing all this down it sounds even worse.
Sorry, I keep trying to get to my point but I get sidetracked (also I'm super ADD, and depression and sleep habits accentuate that). My therapist and I talk a lot about how people with a certain kind of mind/mindset need stimulation, and part of the problem for me is that the work simply isn't stimulating. I've got too much free time (until I've got not enough) and I haven't always done a great job at finding something new to engage in on the side. I picked up a new platform a couple of years ago, which has been a lot of fun to work in (but then I decided to port our platform over to that, which arrested all forward momentum product-wise—like I said, I've made every mistake you can make as a company here...) Maybe what you need is something engaging to start your day with—learn AI (that's what I keep promising myself after this exit happens, assuming it does) or get into some side of technology you've always been less good at. For me that's hardware—I dropped out of my CS program well before we got to the more advanced circuits classes, so while I understand digital logic the EE side of things is a black box for me I've always wanted to grok.
Maybe you just need more work to do—sounds like you've got a lot of free time, thought about consulting work or mentoring at an incubator? As boring a the business may be, I'm certain you've learned a lot of things the hard way, and that's valuable knowledge to share. Maybe if you put something in the afternoon that you really want to do, getting work out of the way early would be easier. But also, maybe you're just not a morning person. If your company is making you enough money, and it really only takes 3-4h/day, is it so bad you don't start til the afternoon? Some people just don't click in til then. Also worth noting, the average american worker only spends about 2-3h/day doing actual work [https://www.inc.com/melanie-curtin/in-an-8-hour-day-the-aver...]
Getting out of the house would probably help some too. Do you have a dedicated work environment? I bought a house recently and one of my favorite things about it is that I have a dedicated office now. (I also get an inordinate amount of pleasure out of yard work, which is difficult to explain. When I'm not fantasizing about bus factors, I fantasize about pulling weeds. Go figure.)
I've been kind of thinking about starting an accountability group or something with a couple of friends. That might be something to consider.
Anyway, I'm sorry to hijack your post with my own problems. I hope there's a kernel of something in there that points you in a direction that makes you feel more fulfilled. Best of luck, friend.