The motivational form of psychological egoism asserts that we always try do what we think is in our own best interest

That statement is true
Psychological egoism believe that even when we choose to help others, we do it because of our own personal reward/benefit.
For example, when we choose to give money to the homeless, psychological egoism argue that it happen because either:
- We want to be seen as good guy by other people around us
or
- We seek the personal satisfaction of 'we've done something good'