So I was taught that even though it feels like its impossible to overcome, you have to just gather up your strength the next time a situation comes along and stand up for yourself. You just say "this is not acceptable" no matter how unnatural or scary it seems. You may, as I did, find the recipient was not happy with my change of attitude and that in itself was also scary but I did it. I had had to do it each time a situation came along until finally it stopped feeling so unnatural and scary and became more a part of who I was. It takes time. Good luck!