Committment, Boundaries.

I have heard people talk about setting boundaries in ministry before (well actually in other things as well, but here is not the sphere where they will be discussed). I have been told it is beneficial for leaders to do so, as it demostrates to less involved individuals that taking on a little more responsibility will not lead to their lives being taken over, or mysteriously sucked away in a huge vacuum (I'm really tiried, so the analogy seems to be appropriate). I have never really done this though, intentionally at least (set boundaries, not being a vacuum). I prefer to be busy, it makes me feel involved, important, useful.
In my second year, I remember being asked to organize a postering party. I should have said no. Instead of feeling like I was asked, I felt like it was expected for me to say yes. I was aware many people were stressed, uber stressed with other things at the time. I was afraid that if I said no, I would look unfaithful, petulant (I over thought it potentially), not desiring more involvement. I became stressed, and did what a think a normal unversity student would do. I called my mom. She quickly identified that through this simple agreement to do something that normally wouldn't have been a big hassel, that I was someone whose choices weren't school oriented, and my desires weren't the same as my parents (something she probably had been feeling for a year before this occassion), and that I was agreeing to do more than I could at the time. She scolded, I cried. I rethought what I should be doing in C4C. I thought about quitting...pretty much all my committments. After a week and talking to two individuals, I was reminded of how valuable this min is, and to drop out completely would be a devasting choice. In the end, I calmed down. This could have been avoided if I had just said no, and trusted that God would raise up another labourer to do what needed to be done, and that I wouldn't have been hated on for focussing on midterms.
Currently I am not stressed out. I am questioning whether it is selfish for me to remove my availability during a time period, despite recognizing that this will create difficulties for others. I have learned that boundaries save future distress. But when are they no longer simply preventative measures, and instead turn into being selfish and inflexible? Aren't boundaries...in their own definition, guarded from being flexibile? Again, I think I am overthinking.