I am one of 3 employees in a very small, very chaotic company run by the founder. He and I have a good relationship (in his eyes), but in reality I have spent 8 years basically building his business and ensuring it doesn't implode each day. (Not blowing my own trumpet - the other 2 employees are part time/freelance, I am the only person who deals with our actual product/purchasing/operations/export/logistics/suppliers etc). Because it is such a niche, and because I've essentially created my role from day 1, it's not going to be a straightforward one to replace.
I've been pretty miserable for a long time, but have held out until now, mostly due to this weird complex I have of "I can't quit because I've poured so much into this". It's of course not my business, but it feels as if it is because I have put in so much. I have worked 6 days a week, 12 hours a day, pretty solidly for 8 years now. My boss genuinely has no idea because he is really just focussed on sales, so as far as he is concerned, a product magically 'arrives' and he just has to sell it. (I am aware how mad it is that I've dealt with this for so long and am aware that I need to work on my boundary setting. But I was actually so enthusiastic about what we were doing/building that for a long time, I was happy to do the crazy hours... I also have share options, although they are basically meaningless until he takes the company public which is very unlikely to ever happen).
So... I have finally decided that I do need out, and have been offered a new job somewhere much more structured/low-stress. I know this is ridiculous but I genuinely am terrified to hand in my notice. Every forum I read about "how to have the conversation" gives advice for situations where there is an HR department / it is a very corporate company / your role is easily replaceable and people saying "just tell him you quit and don't give a reason". This is not that. My boss thinks we are good friends, and we are very close just by virtue of having spent the last 8 years on this thing. I obviously don't want to say "I'm quitting because this is a terrible workplace, you have no clue, you're taking advantage of me, I'm burnt out" etc, because he will not take that well at all (defensive ego!). But I'm worried if I try and make it more "it's not you, it's me" he will just try to offer ways to keep me (which I am not interested in). It is true that a large part of my decision is due to wanting more work/life balance, being able to move away from this area (job/area are tied together), and even if he/the job were amazing I would probably have reached a similar position of wanting to just change my life up a bit.... however if I take this approach I am worried he'll just say "we can make xyz changes and then you'll have a better work/life balance", and I'll then need to tell him why I really want to leave (basically "you're awful").
No idea if this makes any sense outside of the very specific context of this job/working relationship, but I really just need some advice/insights on dealing with this sort of resignation. There is a very high chance that once I go, the company will start to fall apart (again, not bigging myself up - he has just allowed it to get to this point where everything we do is in my head and not easily transferrable). I know people will say "that's not your problem", which is true, but I can't help but feel responsible and somewhat sad about the idea that this place which I have put SO much of my life into could crumble if I walk away (not to mention that he will blame me for that happening).
TLDR: Need to quit, but am in a v small business and deal direct with owner. If I leave, he is likely done. Don't want to tell him I'm going because of his poor management, but if I say it is due to 'worklife balance' he will just try to throw fixes at me which I will then still need to reject. Help!