In his book The Gift of Fear, Gavin de Becker writes that if you say No and people don't take your No seriously, you are being put on alert that they are trying to control you. This control can look like guilt mongering, scare mongering, manipulation or emotional blackmail. You name it, some people will try it and if that doesn't work, they might crank it up a notch and another notch and keep going until you don't know whether you're coming or going and you're all confused.
That's why it's so important for you to get clarity about what you will and will not accept. This is the only way you'll be able to trust yourself and any decisions/choices you need to make and create any boundaries you need to put in place. EFT short cut diagram.
The following phrases might help to start you off:
Even though people get angry at me when I don't do what they want me to do and that makes me feel ... I completely accept how I feel
Even if others pressure me I can still hold my ground and know who I am and what I want (look out for any tail enders/objections here)
Even though others try to make me feel guilty by doing/saying ... and that makes me feel ... I accept how I feel
Even though this bothers me because ... I accept my reasons
Even though I find it difficult to say no/set boundaries because ... (be very specific) I love and accept myself anyway
Even though I find it difficult to follow my own judgement/intuition because ... I accept myself anyway
Even though I question myself (because ...) and maybe others sense that and use it to their advantage, I choose to practice looking after myself
It's ok to say No (watch for any objections here and tap on any that come up)