I had the same problem. I had counselling, and my lovely counsellor sent me off after one session with the instruction to listen to my feelings, all the time, for the whole week. Didn't involve any changes of behaviour, nobody knew I was doing it, and although it was new, it was actually easy.
And it's REALLY enjoyable, after so many years of putting my feelings on the back burner, to think that in every single situation, this was about ME. MY experience, MY needs, MY wants, MY happiness. Wow.
Once you start to do that, the behavioural changes develop on their own. You'll hear the word 'No' come quietly and peacefully out of your mouth. You'll hear yourself say 'I don't like this', and 'I'm not happy right now', and then you'll hear positive things, like 'I'd rather...' and 'Why don't we do it this way instead?'
But don't focus on the script; the words; 'what to say'. Focus on feeling what you feel, and being super, hyper aware of it. The words will come in their own time, and it's a lovely process to go through, watching yourself develop healthy boundaries.
You will also recognise, I imagine, that none of the things you like/want/need, are anything dramatic or unreasonable. They won't be about controlling anybody, they won't create arguments or stress. And you'll be able to bumble along, quietly happy. That's what happened to me, anyway. Hope it does for you too :)