I too find it difficult to set overt spoken boundaries about my needs and desires in relation to acquaintances, friends and loved ones. Like @Adge, doing that has often given rise to less than helpful responses, and frustration and even anger in me. This applies even to the closest people in my life.
I find that people in general are not fond of demands in friendship, even when they are not suffering from mental illness. I guess I prefer for everyone to be free to give what they can and want to, in a natural way. This is partly because I find myself tending to withdraw, perhaps like @greenpea, when demands are made on me. This is partly because I found myself unreliable in various ways, due to my mental illness. It stresses me to feel that I am required to give in certain ways, though I feel I am able to give fairly abundantly, especially on the emotional level, when free to do that in my own way.
What I do find most helpful is acceptance, and I feel I get this from those that are most important to me, even if they don't fully understand the complexities of my mental illness.
Like @Zoe7, I have become more able over to walk away from situations or relationships I find too stressful or unhelpful. I have good boundaries in that way that don't seem to need to be spoken or stated overtly, though the down side of that is that I am probably sometimes hurtful to people who may not understand why I have withdrawn.
I'm guessing this can be quite personal from person to person. Making boundaries may be a different thing for each of us in what form they take. I am possibly a bit too inclined overall to try to appease people, as I care too much really about what others think of me. This is part of my lifelong social anxiety, something I still experience a lot, even now at age 56.
@jess_27, I hope we won't be able to get rid of you, as I find your lived experience and insights so helpful and valuable! You must be a great SANE ambassador and I feel lucky that you have joined us here, at least for tonight.
and I relate to everything you wrote too.