People with serious psychological issues, harmful patterns of behaviour, etc, which affect themselves and those around them... It's so easy to say that you will change. Doing it is another thing.
In your experience, with the people who say "I'll change...I know I can do this...you'll see, I can change. If you just stick around a bit longer you'll see I will change." Do they usually end up changing? How long do you stick around? How much do you let yourself be damaged by their behaviour before you call it a day? What does it take for them to change? Does it tend to help if you - their friend, SO, or whatever the relationship may be - leave them behind after explaining that their behaviour is too harmful? Is that a positive motivation?