I suppose it is always a good start to figure out first if it is an issue with external or internal boundaries, or maybe even some kind of internal plus external issue.
Wether you feel like the space is getting smaller inside of you (preassuring yourself by what you pick up or exhausting yourself, losing capacity due to not realizing thirst and hunger and not taking care of you) or if the space is getting smaller outside (like standing in crowds for example or being pushed to make a decission on the spot). Checking out if you can make out the source of it or not. If there is some kind of inner loss of space like a panic reaction or paralisis involved that makes you highly stressed, tensed, nauseous and reactive so that you cannot see the yes-no compass that way already pointed out.
I like your idea of resentment
@tovlo and its good to read that the idea of giving and receiving fills in
@soulareclipse ^^ I was just thinking about the same. So I'm just gonna add a little different wording here:
Another boundaries marker might be everytime there is an imbalance of giving and receiving. ("Something is too much")
Meaning everytime giving and receiving doesn't come as a natural unity (because it 'feels' right) - but is in a way imbalanced and onesided (A) or an unnatural and intentional tool (B).
A: when you end of in resentment, overcommited or with some kind of feeling of dissapointment due to unrealistic expectations or emotionally overwhelm and empathetic burnout, or a feel ob being robbed, abused or taken advantage of,..(when you are not making yourself an important part of true harmony)
B: actively use giving to get something for example, or even worse make some kind of hidden accounting with an maybe underlining 'you owe me' attitude or some kind of "when I'm nice to you, you have to be nice to me"/"when I'm a good kid you will give me love" sort of direction..
Which actually makes actively training to receive compliments and accepting help and so forth really really important to reach balance throughout life. (Just to mention some weak spots..)
Plus learning how to use language to express boundaries and responsibilities of feelings, expectation or anything else that is picked up. Learning how to have true conversations which is difficult for many introverts but also in general to stick to some basic agreements on how to talk to each other.
Trying to get away from any wording that puts the other person or yourself in a victimized or offending position. I'm not saying that abuse or so should be sugar-coated (God no..), what I mean is that it still shouldn't come at the cost of one's own self-reliance and empowerment (I mean for example not saying "you make me feel this and that" but rather reminding yourself of having a choice to distancing yourself theoretically that comes with a spectrum of "how I could respond to this"/not blindly reacting to anything).
It's not all about distance and brick walls, it's also about letting something happen inside one's own head and creating your reality through the choice of your words (also how you talk to you yourself that is probably often being very hard on yourself). Since the wording is highly involved in a perceiving processes or better said in the judging aspect that is connected to the raw feeling which actually created just then what we call emotions (feelings with some kind of bias underneath it).
I suppose this is a key to learning how to be less reactive and become empowered and chose the right action of responding according to your needs but not entirely cutting yourself off of the group by not letting their needs be part of the deal either - but rather experiencing true moments of giving-receiving unities.
Having ways to get the hang of expressing your own feelings (and needs!) and sorting them (after sponging or absorbing people or losing yourself to mention the extremes) and get out of vicious circles that make you not deal with your own emotions (by distracting and repressing, projecting, intensifying or seperating and locking them away): by writing, making art and music, or having authentic and honest care-givers or later on neutral friends or support who help you understand yourself better during talking, who are also open to information from the iceberg below water and who help you gain clarity and a fresh perspective, but also helping you therefore in trusting in your own perception (pro healthy self-esteem) instead of creating everlasting moments of ambigue realities ("I'm sad but I'll smile anyways", sensitive kids pick up the underlining message and will do the same later on feeding into the circle..). So basically people who don't have an agenda for you themselves (no power struggles involved) but respect you as an autonomous part of the overall oneness.
... ..
end of soul vomiting.