Talking stage is fucked. me (18m) her (18f)

So i have been texting with this girl for about 3 weeks and we keep talking abt hanging out the upcoming weekend. We are both 18. In the end we settled on dinner on a upcoming weekend. A few days passed and i asked her if she can go, and she said she hasnt asked her parents yet. We’ve flirted and told each other that we liked each other many times. Then a few days passed and we were talking as usual over text. One day we were in the middle of a convo and she just ghosts me. I gave it 4-5 days before i sent another text and i asked her if i did anything to upset her and if she needs anyone to talk to ill be here. A week passed and still no response. How could someone just stop replying when i literally sent them a message while they were holding the phone??. Fast forward a week later i see one of my really good old friends post on his story with that same girl i was talking to. I even remember while talking to the girl if she was seeing anyone and she said no. Fast forward to this day shes been dating him for 2 months it looks like and im so lost in myself right now. It seems like when everything’s going good something just has to go bad.

Tldr: she talked to another guy while talking to me and basically used me as a her talking dummy