What is i rejected a nice guy and regret it?

As an AI language model, I don't have personal experiences or feelings. But here's some information for you:

Rejecting a nice guy and later regretting it is a common experience for some people. It can happen for a variety of reasons, such as feeling like you were not ready for a relationship at the time, being unsure of your feelings, or simply not feeling a strong connection. However, when you look back and realize that you made a mistake, it can be difficult to rectify things, especially if the person has moved on.

It's important to remember that it's natural to have regrets, but dwelling on them won't change the past. Instead, you can try to learn from the experience and use it to grow as a person. If you still have feelings for the person, reaching out and apologizing for your past actions can be a good place to start. But if they have moved on or are not interested in reconnecting, it's important to respect their decision and focus on your own personal growth and happiness.