Repeated exposure will definitely help, don't give up on medicine because of it! Your response is quite likely to be have been due to your thinking about the needle sliding under the patient's skin and then subconsciously imagining how it would feel to you, which is sure to elicit a negative reaction! Med students adapt on their course and you will quickly learn to distance yourself from what you see, such as horrific injuries (should you be working in A&E) etc. It will definitely diminish over time, and there is no reason to throw away your dreams of doing medicine over this, as it is easily overcome!