Yes, I relate, too. If I were to put words to my process, before I was flooded, overwhelmed, and disabled by trauma memories, I had boundaries that were very black and white, that were rather rigid, and they would either come out of me in a flare or a wimper, and I had no real ability to dialogue.
The breaking of my old ego (means by which I interface with the world) structure, from trauma memories, left me lying flat, not able or knowing how to be an advocate for myself, at all. No self-confidence, since all the old tools I knew failed me.
As I dealt with my memories, I simultaneously rebuilt my ego structure--basic positive self-confidence, rebuilt my ability to speak without getting short of breath (old trauma patterns), developed my emotional and feeling skills, so that I knew what I was feeling, and so that i could speak from MY perspective.
With that foundation built, just in the last 3 years have I been able to notice and approach, when I needed to set boundaries. And only in the past few months have I been able to actually set them. There is still a delay from when I notice I have been violated and when I set my boundaries, but I am doing it, respectfully, and non-violently.
I practiced a lot, each of the steps above (feeling the violations (small and large) in daily interactions, rawly expressing my feelings, practicing formulating and non-violently speaking the boundaries I wanted. Gathering a few friends that could support me after I experienced panic, after speaking my boundaries, was really important.
As mentioned above, there is always a risk in stating your "wants", and if you do it in a way that is respectful and invites dialogue, rather than closure, you may minimize your risks, and even deepen your friendships.
So you are in good company. And, like me, i think you will be able to state boundaries. Just practice, starting small, and build supportive friends.
Taking some Linklater Voice Technique ( find it in acting schools, and non-actors can take them) classes helped me a lot (for two years)-it teaches you how to relax everything, so the words can bubble out, freely. A self defense class gave me the right to have physical, and hence, psychological boundaries-Model Mugging. I still take the class once a year.
Instead of your therapist pointing out that you need better boundaries, ask (always risky to ask someone to change their style) if they would help you formulate the skills to have better boundaries by asking you to consider, "How did you feel about aomeone else's actions?", "What boundaries would you like?", "If you could non-violently (istatements) speak to a person about what you needed, what would you say?" If your therapist doesn't want to do this, you can ask yourself the questions.
These books were helpful, 'Focusing Technique', 'Speak Up, Speak Out', 'If You Meet The Buddha On The Road Kill Him, and Non-Violent Communication Technique literature and classes were helpful, and Mindfulness Meditation was useful for self-calming.
Also, sometime the old triggered feelings need to be rawly expressed-in therapy is the safest-if your therapist is comfortable, or in a car, or by hitting pillows.
Hope this helps. :)