People don't change. So many women/men who come into my office thought they could "change" their spouse. It just doesn't happen. Even if they WANT to change, their core personalities are what they are.
The same thing applies to all relationships. How many time have you said "well, she is not like that with ME". If she is disloyal, mean or disrespectful to her other friends, it's only a matter of time before she does it to you. You CAN and SHOULD judge a person's character by the way they treat other people. I hate to bust your bubble but you are not that special.
Next time you think you can change someone remember a scorpion is always a scorpion.
THE SCORPION AND THE FROG
One day, a scorpion looked around at the mountain where he lived and decided that he wanted a change.
So he set out on a journey through the forests and hills. He climbed over rocks and under vines and kept going until he reached a river.