WakeUp is right in suggesting a time limit but only you can determine what it is.
How to tell her: "I respect your need for space and a break. I'd rather not go nuts trying to figure out what 'a break' could possibly mean so please consider these guide lines. Commitment to me even though we are on 'break' status. I do not consider this a free pass for you to test the waters with someone new. This would be a deal breaker for me. Also, we need a time limit on this, I am not going to wait forever. After (amount of time you're comfortable with) you can work on yourself with or without me. I won't spend forever in limbo" Of course you can soften it.
But you probably won't do this either. You'll let her string you along until she moves on. Then you will post here about how miserable you are without her and ask us what you did wrong. We will refer you back to this thread. Save yourself that humiliation and actually accept the advice offered.
(Wow. I'm a bit of a bitch today....)
'People are never perfect but love can be. People waste time looking for the perfect lover rather than creating the perfect love' - Princess Leigh-Cheri from Still Life With Woodpecker.