Do you ever feel like you do not know where you begin and another person ends? Are you the rescuer type? Do you constantly feel worn out and suffocated? Boundary setting is something you may need to work on if you can identify with those statements.
First of all, when you try to set boundaries, do not do it when you are angry. You are more likely to go overboard and set your partner a task of cleaning the kitchen every day for a whole year (if you know what I mean!!) It is helpful to use very few words and be specific when you communicate so he or anyone else you're working on boundary setting does not feel attacked. Avoid rationalising, and apologising. Do not feel ashamed or afraid when you set boundaries too. Also, learn to listen closely to yourself. Do not let the barrier of shame restrict you from taking care of yourself. If you feel victimised, suffocated, or threatened by others, you need to pay attention to what your body is telling you.
Sometimes, others may not like the new "You" because they feel defeated and may not be able to manipulate you or push your buttons. That is their issue and part of boundary setting is not taking that on as your problem too.You may want to specify consequences and give ultimatums in order to enforce the boundaries - what is it that is and is not acceptable about your parent/partner/friends behaviour?