Can you give an example of what you have tried? Boundaries can be very tricky as there are a lot of variables involved depending on the people involved. To put it simply though, boundaries are about us and what we will do to protect us while also acting in ways that are consistent with our personal values.
Setting Boundaries and Setting Limits is an article we have here that explains what I am talking about when I mention personal values.
Something that might help is to first learn how to communicate in ways that can help prevent dysregulation or at least not make things worse. Learning how
not to invalidate is important as is learning to
not JADE (justify, argue, defend, explain).
EDIT: sorry, I hit post too soon. Read the articles, ask questions, see how you may apply these things in your own relationship. Learning not to make things worse will help you to settle yourself and make it easier to implement boundaries.
Let's talk about this and see what we can come up with.