So many of my clients deal with boundary issues. I deal with those myself plenty of times. A lot of what we confront on a daily basis comes under the umbrella of "boundary issues."
When someone misses the deadline of when they were to have a promised assignment to me and still expects the same quick turnaround I'd promised when I thought I'd have it earlier, that's a boundary issue. If this happens time after time and I say nothing, I'm teaching that person that it's OK to disregard deadlines with me. I'm teaching that person that it's quite OK to take their own sweet time getting something to me and still expect that I'll do a quick turnaround.
When a person committed to a date and time with me and didn't show up or even let me know they wouldn't show up, that's a boundary issue for me. I need to set my boundaries of what I will accept and what I won't accept. And that's not to say there's no grace when emergencies come up. That's different!
When a friend regularly puts me down or makes fun of me and I say nothing but am deeply hurt by it, that's a boundary issue. It's up to me to speak up.
When a colleague always insists on "my way or the highway," that's a boundary issue. It's up to me to say what I want and need. If I don't say so, I have no right to complain later or to be resentful.
And when I constantly give advice to others (when it's not sought) rather than tending to my own garden, I have a boundary issue I need to examine.
Look at how well you keep boundaries. Need to make some changes? And be aware of things in your life that cross your boundaries. Perhaps you are doing things for others that leave you resentful and angry later. Check out what's really happening. Do you need to set some boundaries? Do you need to say what you want and need? What you will or won't accept?