Why am i scared to kiss my boyfriend?

This always happens, everytime I get into a relationship I act shy to kiss I'm not shy about anything else just kissing !. Ive kissed plenty of times but this always happens on the 1st or 2nd kiss &I don't know why but when I kiss I feel kind of stupid bc im always thinking to hard & picturing about how i look and then ill think about if he's thinking the same thing and I wouldnt want to kiss him . How can I stop thinking that ? How could I fix this problem? SN:I'm a junior in highschool