You very probably won’t lose her as a friend if you tell her you don’t want a relationship. It’s just the guys that have the kind of sex drive that forces us to cut off contact with women we want to sleep with that don’t want to sleep with us.
Just be honest and you’ll be fine. What you don’t want to do is mess her life up by leading her on. That’s worse than a rejection and if you really value her as a friend, you’ll let her make a fully-informed decision.
What I would advise is to not get too specific about why you don’t want a relationship. From experience I’ve learned that there’s just no way for that to be a useful discussion, ego just gets in the way of communication. Be nice, tactful, vague, and positive.