I think the hardest part about break-ups is separating from the attachment we somehow feel for the person. We grow fond of people once we get to know them. But, if we know early on that there are red flags that makes someone very incompatible with us, I think it's smart to make decisions most suitable for both partners.
I think people can change- their habits, attitudes can change, their beliefs, values, tastes in clothing, music (given the environment/life situation). In terms of their personality, I think it pretty much stays consistent all throughout their life. Not unless if they have a head concussion, or develop Alzheimer's or something..
Berb- I don't think you should be so hard on yourself. You did give the guy a fair chance. It's not like you made a snap judgment about him and not give him any chances at all. Kuddos to you for putting yourself out there. And I agree- even for someone who does the breaking up, it hurts as well, especially when we don't want to see others hurt, or watch something that 'could have' been something that's just 'not' what we're looking for. It's beneficial to be honest with ourselves/to the other person early on.