How to lose the spark

Don't get me wrong.
Brief background, I'm happily married with 1 kids but due to work we are unable to meet on weekdays. I'm also currently pursuing higher education as part timer.

So, I got to know a girl in my class about 3 weeks ago. My intention was only as study/discussion friend, and we text a lot but no flirting. Then one night we were just casual chatting about life, study, career over a dinner (1st dinner, not date). We shared a lot but nothing intimacy, and on the way to drop her back, we stopped to chat in the car. She told me about her love life and family, seeing her almost in tears I teased her and she broke out with a laughter. We just joked the rest of night, I even causes some pain to her injured finger (she cut herself 2 days before) and I apologized after. It's getting late and I drove her back.

After that night, I can feel she had kinda develop some feeling and I also keep thinking about her. I realized I'm in trouble.

She knows I'm married, and I think she also felt she went overboard as the incoming text were greatly reduce and very normal (no more teasing). Yet her text still somehow get me some butterfly...

My big question is as titled, how to lose the spark? But not losing the friendship.