I made the mistake because of an out-of-process change. You must always do another QA run through after a change has been made. We started using a change log for the emails after this incident.
THE FEAR is real and is so for good reason. It keeps you on your toes and makes sure you test and test again.
Team support and communication is crucial to make it in this line of work. I was lucky my boss understood these kinds of mistakes aren't uncommon in our industry. However, if he didn't have that kind of knowledge, it would have been my job to show him just how hard email marketing can be.
At my current company, a few months ago, I sent out an email that read, "Hi [Customer Name Test], in the body of the mail instead of the actual person's name. This is a common mistake, but not one that had been committed at my current place of employment. Being the first one to make that mistake was not fun. The email as sent to 20k people, so not that bad, it could have been worse. However, since we're B2B, there were possible financial implications that might have hurt us.
I would look into Trello + kanban method. Asana would be more robust and flexible, but it has a steeper learning curve than trello.