I'd have to second Michael in that you want to establish good communications with your agent so that everyone is on the same page and you both have well defined expectations for the relationship. That said, I've *fired* a few clients knowing that we weren't working well together, and recommended agents that fit their style and values better than I.
This agent has showed you several homes, if they are not a close possible match with what you've told the agent you want to purchase, I sense you have a mis-communication going on that you should definitely bring up in conversation. When you find the right Realtor, you won't want to work with anyone else. Good luck!
Hopefully you have ethical reasons to change repesentation and have offered the agent the opportunity to correct the situation.
Of course you can. The agent will have been disappointed about spending so much time with you and not seeing a result. What did the agent do? Is this something you can talk to him/her about and rectify? Can you try that first? If not, the simple answer is yes.