Talking to a girl on the phone

I’ve\[19M\] been chatting with this girl\[19\] since few months, we shared so many things about ourselves and we get along pretty well while texting each other. Anyways, i asked her to talk on the phone and she agreed. Here’s the problem: I’m not so good at communicating since i have social anxiety, i don’t even know how i managed to have a 1.5 hour long conversation with her. She was giggling, lauging and having a good time despite i was a little bit awkward. I know that’s a victory but i feel like if i had better conversation skills we could have talked for 3-4 hours. I wasn’t so nervous or something like that(which i am still amazed by) but my mind went blank a lot of times. What should i do to solve this?