Hello merry -
Ask 101 people and you will gain 101 different responses - some of them similar - some of them contradicting.
Personally, I would suggest a person who does not choose to submit on a regular basis but preferes to only for play purposes is not submissive, but a bottom. Submissives do not have to believe in TPE - but will be submissive in more than just the play area. I am submissive to Darcy, but will Top him on occasion during sessions - but that definately doesn't make me dominant.
Hope that helps, and is my opinion. Others may vary...[;)]
What you could ask yourself is why is it getting to you? Do you feel you need a specific label, or that not being 'a submissive' is a step down? Then you can try and work on that...
Peace