If you can't be yourself around that person, just don't be around that person. Simple enough.
So there is just one "yourself" that always must exist in every relationship that you have?
Are you the same "yourself" around your family that you are around your friends? Co-workers? Is "yourself" completely unaffected by the environment that it exists in?
Understanding that you are likely more creative than you give "yourself" credit for will make this fun rather than frustrating.
In plain old laymans terms , That person is a control freak and you have deep down hidden fears of them , like OMG , What are they up to next ? .