It amazes me how many people in our industry will eagerly work seven days a week without pay for an employer that:
A) Won't do it themselves. B) Can easily afford to pay for it, but won't.
It won't help you get ahead, either. It shows gullibility, not dedication. That's not the kind of quality you want in a leader of anything. It's for this reason I would disagree with your premise that by not allowing yourself to be exploited you're forgoing future career success. You've learned the first lesson in success which is, crassly, don't be a punk.
Oh man, I've recently gone back into contracting and absolutely love this aspect. Sure, contracting has its downsides, but if you don't want to deal with being bossed around contracting is brilliant.
Then, no matter what crazy things are going on, he's insulated. As he says, "full time is full time, but 32 hours is 32 hours."
I did this early in my career for a few years, and it was absolutely worth it for my career in the long run. It wasn’t a one sided deal either. My boss allowed me to have all sorts of responsibilities that at the time, I absolutely didn’t have the experience to back up. I got through it entirely based on perseverance and hard work, and when I left that company I landed a much nicer job that was years ahead of what I would have otherwise been able to get. Now I’ve learned and done enough that I largely get to set the terms for my own employment. I haven’t even had to properly apply to a job since then.
It worked for me at the time, because I was young and didn’t really have any responsibilities outside of work. Situations like this are only really a problem if you’re not getting something bigger out of it, or if you have too many personal responsibilities to manage it.
You only think of yourself as weak, because you compare yourself to people working 12-15 hours every day for years, because they want to "make it" in the software world. In the end, most of them are going to be burned out and depressed, with no family or friends around, because they simply didn't have any time developing that part of their lives. Hopefully, some of them will continue on to do that then later in life.
I'm okay with 7 hours of sleep every day and can function well for 8 hours of work in the work week. But then I need time on the weekends with my friends and with myself to recover. When I'm doing a lot of overtime, the first thing I do on Friday evening is come home, get out of my shoes and fall onto my bed. It's completely normal to be tired after 8 hours of work and needing sleep. Our super-fast, ever changing society puts a lot of pressure on us, and at some point, people will not function under that pressure anymore. Which is why mental health issues are on a high rise.
You may just be getting older. These started for me at 40. And it's not just the physical symptoms: the multiple days of depression and low motivation after they've subsided are deeply unpleasant as well. Drinking to excess just isn't worth it any more, if it ever was.
They (we) earn an above average salary, at maybe non Forture 500 companies, 40ish hours weeks, still enjoy what they do at work, but can find fulfillment in other domains outside of it.
It's a matter of what success means to you. I don't value a fancy job title or a higher number on my bank account (above the 'basic' software engineer pay grade I am at already) enough to do this kind of mentally back-breaking BS work. I do the job I was hired for and get paid for. I do that job to the best of my ability 9-5 (well, 8-4) and then I leave.
For me, that constitutes having succeeded at the school, uni, job hunting and employment challenges. I expect most future successes in my live will come from my family/social live or will be personal triumphs like getting into shape and learning interesting new skills.