How do I tell this girl how I feel when She has a boyfriend?

Here is my situation,
There is this girl I like that so happens to be the cousin of my best friend. Now, we are the same age (17) so graduation is getting pretty close. I have came to the conclusion that I’m absolutely crazy about this girl and, I will do literally anything to have her.

Unfortunately… She has a boyfriend that treats her like absolute garbage. I don’t have the luxury telling her how I feel face to face since I no longer go to her school. I still live in the same area and hang out with her whenever she’s with my best friend. I can never find a perfect time to tell her since I can never get her alone.

I’m not worried about whether she likes me or not because whenever I text her she’s flirty. Also for this reason… One Day My mom, Me, my friend, the girl I like, my friends mom went out to have some fun. Apparently, when I got up to use the bathroom at lunch, The girls aunt told her I liked her and I would treat her right unlike her ex. No one told me about this conversation so 2 weeks later my mom told me she blushed and was hitting on me the whole day. (Laughing at my jokes, Sitting beside me, etc) I had an idea that she was hitting, but I didn’t flatter myself since I knew she just got back together with her ex.

After A few weeks of living emotional hell I finally got enough balls to tell my best friend I liked his cousin. He told me “I had a feeling, It really sucks for the both of you since she’s with (Insert Name). Well, I can give you her number.” So I said ” No man. Give her mine… and if she wants to text me, she will.” 30 seconds later I get a text from her. So I text her back and forth for 5 hours straight. Then for the next few days we text like crazy (She’s very flirty with me) and now it’s gotten to the point where she isn’t starting the conversations. Now I’m waiting for her to start them… So when she does I’ll need to tell her how I feel before she loses what little interest she has left.

Now here’s the situation. I can’t tell her face to face or otherwise I’d do that. It’s either call her when she’s alone or through a text. (I don’t want to text her if it would hurt my chances) Keep in mind, She has a boyfriend, but I can’t hold this in any longer. What would I say? I absolutely DO NOT want to screw this up! I care about her more than life itself. I would wait to see her face to face but that could be a few months due to the fact I only see her when she’s hanging out with my best friend at the same time as me. What should I say? When should I say it?

As you can tell by how long this is… I really like this girl. Sorry for any grammatical errors. I also felt the extreme detail was necessary.