Emetophobia doesn't just 'happen'- it does not exist in a vacuum. Not only can one specific event start the process of getting this phobia, but there are othervariables which serve to reinforce it, or that subconsciously make it an attractive option (ie- lack of control in other factors of your life can lead you to want to control bodily functions- including vomitting, which can be scary because it can only be controlled to a certain extent).
By discovering the factors which triggered and reinforced your emetophobia (whether that be family stress, problems relating to school, relationships, etc.), you can see how how those types of situations affect you, and learn better coping mechanisms. Although it's easy to just think that emet is the problem- it can really be the end result of different types of problems, and discovering/coping with those will help. You should definitely help the what (ie- treat the phobia itself)- but the why needs to be addressed as well.
I know for myself that my emet comes on strong when I am stressed about work or school- by knowing this, I can do things which limit this stress (start assignments earlier so they dont wait until the last minute, etc.), and learn ways to control that stress so I just don't automatically turn to my fear, or let it come on strong. It really has helped 
I suggest that you talk to your therapist about it, so she can explain why she is taking this approach. In fact, if you have any questions regarding treatment or her approach to it, always ask- it's better to get an answer then wondering why she is doing things a certain way, or if it's really going to benefit you.
*amber*
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