Procuring Cause is more than simply "the agent who first showed you the home"... it is an unbroken chain of events leading to the purchase of the property".
Firing the agent would certainly break the chain of events... and would "probably" entitle the showing agent to "nothing". (I say probably because they might take you before a mediation board, who might award them a small "showing fee" portion of the commission... but either way, it would not have to come from YOU (Julie) the buyer.
This is one of those cases where the buyer (client) gets what they want. You cannot and should not be forced to work with an agent simply because they showed you the house. Especially if you no longer have confidence in that agent. Explain that to the agent's managing broker, and ask her to assign a new agent to you to write the offer. The managing broker will then work out the logistics of who gets paid, and how much, behind the scenes, and it should have nothing to do with you or your purchase.