It's good to have a critical eye in the early stages of dating. Some people make the mistake of being so focused on getting the other person to like them or trying to gauge if there is chemistry, that they forget to make sure the person is good enough for them. This is different from putting up a wall, where you trust noone and everyone is guilty until proven otherwise. You can be open to love, but still mindful.
I also think it's rare that someone does a complete 180 personality-wise. Usually there are warning signs along the way but you choose to give that person the benefit of the doubt because they might meet your criteria in other ways, looks, chemistry, personality traits, etc.,
Next time if you start dating a woman with all these hoops to jump through, your spidey senses will kick in. If you put up a wall, you're only seeing your past relationships as mistakes and not valuable experiences to learn from.
“Inside every cynical person, there is a disappointed idealist”--George Carlin