One other thing about boundaries that occurs to me, and sometimes a difficult step for codependents.
Sometimes other people will get mad at you when you enforce your boundaries. For instance, for years I was very upset by my husband's love of pornography. I do not like porn, I think it is unhealthy, and he was very into it.
We had a long talk this weekend about taking next steps to move on, since he has begun seeing someone and spent the night with her. We talked a little about the porn issue, and he got defensive and mad, trying to tell me I was silly to have a problem with it.
In the past, I would have apologized or felt like I should do something so he wouldn't be mad about it. I would have questioned my boundary. That's so typical for me as a codependent.
This time, I just said "I don't need your permission to have a boundary. They are MY boundaries, and just because YOU don't like them doesn't mean they don't exist for me. Be mad all you want, I don't owe you an apology for disliking porn."
Remember that it is OK for others to be angry. I don't have to own their anger, control or fix it. They are responsible for their own feelings and I am responsible for mine.