How do you continue to have confidence when you are rejected?

I don’t.

Evaluating your self esteem on your “success” with one girl is a fool’s mission.
Girls are great, but to hold yourself hostage to their capricious whims invites disappointment. In a post modern, industrial epoch, it is the wrong measurement standard.

You only ask her out once and if she says no, you move on.

That is something I cannot tell you, it is an individual thing. What works for me, may not work for you. I can only tell you is how well your esteem weathers is by what benchmark or criterion you validate yourself by. If I were not validated on a true foundation and one Rock solid, this place would have had me running away feeling like I had a problem.

It might not have anything to do with you. She gave you her number but in the meantime, things have changed her mind – like she met a guy she considers to be gorgeous. You can internalise her decision not to date you and see it as a flaw in you, or you can see it as just not being in the right place and the right time. Move on. There are millions of beautiful, lovely women. Find the one that’s right for you. She’s out there. You won’t find her sitting at home licking your wounds because one girl didn’t think you were the man for her right now.

^ [...things have changed her mind – like she met a guy she considers to be gorgeous.
Oh, she cannot do that, that would make her soooooo shallow not to look at a guy’s wallet personality before deciding date him or more ~~~

You try to remember that there are about 3.500.000.000 women on Earth. How many have you asked, and how big a percentage is that?

Practice.

I’ll keep telling myself I’m not looking for the best person for me, I’m looking for the right one. If he/she’s not, then I’ll keep looking.