i can understand the difficult situation that you are facing. Having low confidence in yourself can sometimes make other people take you for granted and not consider your feelings when they say/do certain things.
In situations like these its best to Respond than React.When you react its impulsive and you just say things out in anger which will only further complicate things and portray you in a bad light.But when you respond, you take a step back, think about what to say to the other person and then present it to the particular person at the appropriate time and place. This will help you to convey your thoughts and will also make the other person understand your perspective.
You can also start working on increasing your confidence level with role playing .This will help you prepare to deal with situations where you feel you have trouble responding .This in turn will help your overcome low self esteem issues.
Hope this Helps.