How to remove part of a design

I have a design where I would like to completely remove part of the border with an angled edge. I have tried using the pen tool to create a vector over the area I want to remove and then removing it using booleans but none of the options seem to work. I just want to erase part of it but can’t seem to find a simple way to do this. Can anyone tell me how I could achieve this look?

This is what I am trying to achieve. The border connects all the way around and in this pic I have just used some white rectangles covering them to try to show you what I’m after.

Any help would be much appreciated 🙂