How can I prove to this girl I really like her and that I'm over my ex, and I want to move on and be with her?

My ex is alittle "clingy" to say the least. She wants to be best friends and stuff but I really don't want that, she hurt me pretty bad. Then I met this girl ooo man, I soon forgot all about my ex, and me and this new girl started hanging out and having fun, and we fell for each other..I really like her alot and last night, me and her were hanging out. My ex gf texted me and said "I Love You =)" which she does every now and then, because we are "Best Friends" or whatever. I wasn't going to text back because i was with this new girl....but she saw the message...now she thinks i'm going to up and leave for my ex, and she thinks i still "love" my ex. I don't want my ex, i want to be with this new girl. How can i prove to her I am over my ex?