I (19M) need advice on how to approach this girl (18F)

I have known this girl since my sophomore year in high school. She is a year below me so she is still a senior in high school while I am a freshman in college. We had mutually liked each other but she had gotten into a relationship with another guy in my junior year. She has been on and off with another guy after that relationship and is now finally done with the most recent one as of about a month ago. I text her every so often, make sure she’s alright if she’s sad on social media, took her to my senior prom, etc. but don’t know how to approach her on attempting to ask her out. Obviously now is a tough time but she just sent me pictures from senior prom a year ago and put one up on her social media story of us. Trying to figure out what to do here, any advice helps!