i have a stepmother who my dad married when i was eight. i resented her for years and to be honest i still do a bit, she made me call her 'mum' and said things like 'i'm your mum now' (my mum died so i just found her saying things like that totally rude) and the worst thing i did was to not say anything. make sure you don't agree to things you aren't happy with, keep the boundaries etc. and keep your memories with your mum, my stepmother throws a strop if my dad or i ever mention anything that happened before we knew her, god forbid i should ever mention my mum in the house. i recently told my stepmother i didn't want to call her mum anymore (took nine years to pluck up the courage) and am just so relieved. if you are close to your mum, talk to her so she knows how you feel, then she will be aware of that when saying/doing/arranging things. good luck.