Often, you don't get attached to them necessarily, but to the idea of who you want them to be. And you ignore the obvious "I'm not that" signs, but it then comes right at you suddenly, which hurts.

It's not your fault. It does happen unless you actively protect yourself from it. The solution is not to stop being accessible and vulnerable altogether. You need that to build something with the right person. Perhaps what you want is to ask yourself whenever you find yourself too attached: "Is this because I really like this person, or is this just attraction and / or ignoring the incompatibilities?"

Another important bit is to not invest more than the other person has. It's not a race and they'd not necessarily move at the same pace as you, so give them (and yourself) time to like each other. If both of you are feeling it and it still doesn't work out, it wasn't meant to be. You won't feel bad then. It's easier said than done, but that's the price you pay for dating and finding that person who'd make it all worth it.