Sex is intense...

I recently started having sex with my boyfriend, and it sounds silly, I'm shocked by how intense it is physically and emotionally...I have to take deep breaths to keep from trembling sometimes. Maybe I wasn't ready to get this intimate? He didn't pressure me or anything. I'm not sure how to deal with this, he's my best friend but now I feel overwhelmed by how close we're getting, like I almost feel the need to break up.