I understand how confusing it is when you want to assert yourself, while not appearing angry or upset. I think it depends on how you assert yourself, for example the tone and words you use. Anger actually can make me be assertive as I tend to be passive, but I try to wrap it appropriately to appear acceptable to others. It works sometimes.

You were working and have experience, so although you may start as a temporary employee, probably you won't start at an entry-level position. In any case, it's how you present yourself that how others will treat you. How do you assess your self-esteem level?

May I ask what is your aim of obtaining a master degree?