I cant get her out of my head. Am i infatuated?

i finally told the girl of my dreams that i like her but i didnt say it to her face. i wrote her a letter and put my phone number on the bottom. she replied by texting me and telling me that she had a boyfriend but thanked me for the offer. i rather stupidly texted her back and told her that i had fell in love with her and i hardly know anything about her. i hate to think i have upset her and i want to apolgize to her for what i said because i think it upset her. should i take her to one side and try to tell her how sorry i am?. i still cant stop thinking about her and it makes me feel so awful to think that i have upset her in anyway. what should i do???.