I mix up names all the time. So much so that it's become a bit of a joke. It was particularly memorable when I was new with new bf (now hubby) but called him by my ex's name a few times. Unlike your love interest, my bf just laughed it off. I can't tell you why it happens - it just does.
The inability of your new girl to laugh it off makes me think that she's either really insecure, takes herself far too seriously or lacks a sense of humour. Or a bit of everything. Yeah, it's a tricky mistake to make....but it's also showing up the new girl in a not so flattering way.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.