I would suggest you meet her in person BEFORE you ask her to the prom. Say something like "I really like what I see of you online and I'd like to get to know you better. Could I meet you in person sometime soon?" If she says "yes", then go right in and figure out a time and place which suits you both. Don't leave the 'yes' hanging to be acted on at a future date because you may lose courage to go back in.
If you start hanging out with her and seem to have a rapport, then it's time to ask her to the prom.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.