Remember that you are worthy
You may have unconsciously trained yourself to silence your own voice in order to put the needs of others first in developing a "selfless" mindset. You'll need to devote time and effort to understanding your own requirements and assuring yourself that your voice matters. Self-acceptance is an important part of this process, but it can be difficult for persons with low self-esteem to achieve.
Start a fresh cycle of self-acceptance instead of a cycle of people-pleasing that makes you feel worse about yourself. Keep track of your progress: make a list of the times when you were able to assert yourself successfully and look back to those notes when you need a boost. You could want to develop a mantra that helps you in difficult situations: affirmative statements such as "I CAN say no," "I am worthy of my own time," "my opinion matters," and others could be helpful