Being the sole person responsible for a system that customers rely on 24x7 to make them money is just about the most stressful thing I've ever experienced as a developer, so I can definitely relate to that weight on your shoulders.
If it's any small consolation, I'll bet you beat yourselves up about the downtime issues way more than your customers ever would.
Actually… I didn't really beat myself up about the downtime. I am totally over beating myself up for things… it just makes it harder to fix them and it's not as if my guilt would help anyone else feel better!
It was upsetting, though, on an intellectual level. I wanted to do it better.
If it's any small consolation, I'll bet you beat yourselves up about the downtime issues way more than your customers ever would.