Rejection sucks, and for some reason, rejection from acquaintances and strangers especially sucks. Maybe you submitted a proposal to give a talk at an event and weren't selected for it. Maybe you applied for a job and they didn't give you an interview. Maybe you asked for a promotion or salary raise and couldn't get what you needed. How do we move on from rejection like this?
First of all, don't immediately take it personally. You're definitely awesome, but let's take a breather and analyze the situation more closely. There are more than likely some good reasons for the rejection from the other person's perspective. Your talk topic may not have aligned closely enough for the event's purpose. Another applicant may have been more qualified for the job. Your company might not have an open position to promote you to or enough money to raise your salary.
Before you ask any follow-up questions, make sure you genuinely thank the person who rejected you for their time. Doing so will increase your chances of them keeping you mind for future opportunities.
Once you've thanked them, then consider some potential questions for feedback. You want to make sure they don't sound bitter, so if you want to double-check your wording and tone, see if you can find someone to review them before you move forward. You can ask what would have made your talk proposition more desirable, what skills or qualifications you were missing from your application, or what you can do at your job to be more seriously considered for a promotion/raise next time.
This is where you can take the answers to these questions and run with them. Hopefully, you receive positive, helpful feedback. Build on it and make yourself an even stronger candidate. If the feedback wasn't helpful or you get shrugged off, however, it might be time to look deeper into the situation. In this case, find an outside perspective to talk to about it. They may see something you don't.
Regardless, keep in mind that you are not alone in being rejected. It happens to everyone, every day. Don't be afraid to put yourself out there. You might come across the opportunity of a lifetime.