to love someone set her free ......
i don't understand why you want to tell her and chase her after you "realise" your feeling. you're not a hunter.
besides, why you ask such question sin? you scare of losing her, even just be friends, or you don't think she will leave her bf and turn to you?
i think after you asked the question, you should have the answer.
1. you have spent more time with her and found some of her good points. of coz, you're happy with her accompany, the you THINK you love her. love is something not from "realise"
2. you don't have confidence. simply that girl is very stable with her bf. atleast you don't see the girl "realise" she loves you.
tresure her and backing her up no matter what happens. she will realise such a good guy is with her all the way. then .............