I hate to sound trendy and cool, but let's unpack this. When was the last time you said no? Did you offer an excuse or a white lie to feel less guilty? How much do you believe that you need to put your oxygen mask on first before you can help others?
Oh, and what they hell are boundaries and how does one "set them"?
The answer is that boundaries are unique to you and sometimes we don't even know what they are much less how to set them.
Therapy with me will help you figure them out, get some phrasing to set them in an assertive and not aggressive way, and the encouragement to maintain them.
BONUS: When you have boundaries you are also modeling to others how to do that in their own lives. They will thank you later.