Hey man,
Im guessing you understood your first mistake, you simply waited for too long..
and you know you should believe that if a girl liked you once then why not twice?
it was just a mistake of letting it slip away because you waited too long to have a relationship or to take things further.
If I were you,
Id simply try to hang out with her more like before and show a bit more of interest. Just like you would do with a new girl, tease her and get closer. But show her you ve changed a bit, that you re not scared of taking risks and show that you want her. (Maybe she thinks you re not interested anymore) and Here you have an advantage of knowing her, so you should know what she likes and what she doesnt. use it to your advantage so make her smile, let her feel again what she liked when she was around you before..
keep goin man!
