Heartsick works when you're all mopey &"Mehh I miss him" but you're most definitely nothing remotely broaching anything near 'in love' cos that's just ridiculous cos you're 15 and far too young to be in love with anyone but Mark Owen (cos he's adorable and would never break your heart ((and also blatantly gay)) ), and also because just no, love doesn't happen to down-to-earth girls like you.
"Aw, you're lovesick."
"What's another word for love?"
"I'm heartsick. Love is too strong a word."