My 12 year olds turning into a nightmare what do i do?

ive been having more and more arguments lately with my daughter whos just gone 12,she started her period a couple weeks ago so i kno hormones are alot to blame but im starting to dislike her alot of the time thesedays as shes being very aggressive towards me,it dosnt help that im 8 months pregnant also so ive hardly the energy to argue back,i seperated from her dad a few years ago and i kno this was tough on her,im now with a new partner who she gets on very well with actually they get on better than me and her as he plays more of a friend role with her.its like if i even ask her to clean her room she bites my head off and the looks i get sometimes are horrible,i really do my best for her she gets alot o9f time and attention and im always buying her clothes etc..she wants for nothing at all,ive also a very good family who dote on her,shes always been taking on holidays etc..i spend lots of one on one time with her also we go shopping quiet alot together but thats even gotton to the stage where i dont want go with her anymore as we end up arguing in town.shes excited about the new baby and spends alot of her own money buying baby things,and she can be very good like bringing me tea in bed,but mostly she acts like a spoilt brat throwing tantrums when i refuse her something etc.im worried now about what things will be like when the baby arrives,im gonna b afraid to show the baby too much affection in front of her,already i find myselftrying not to talk about the baby too much and its hard as its my partners 1st and we are excited,i kno she mite feel left out but im actually exhaused from trying to make sure shes not shes even came to all our scans.i love her to bits and shes always been my priority but i want to b able to love my 2 kids equally without her feeling jealous or left out.i can see things getting worse with her in the nxt few years but i dont kno how to handle it,i want the best for her and i dont want her to end up hating me,my partner says i spoil her and do too much for her but i feel so guilty over how my relationship with her dad ended i try to overcompensate with her.shes my 1st baby and i want to just hav a happy home life now and to get on with her without biting each others heads off,the baby deserves to grow up in a happy enviroment also,what do i do about her?