How can i stop my lenses hurting my eye?

ive just started using contact lenses- im on the 4th day of my 10-15 day tester lenses to see how i get on with them... i have no problem with m right eye but every time i put the lens in my left eye it either hurts so much i have to immediately get it out or it feels like i have an eyelash in my eye. i eventually get it in so that it doesnt hurt but it takes a lot of time... how can i prevent this happening?? thanks