When people are repentant, they’re feeling or showing remorse. They're sorry and ashamed of something. If you take the last cookie and then lie about it, you’re probably going to feel repentant later, if you have a heart.
If you’re feeling repentant, you’re ready to apologize because you know you did something wrong. It could be that you kissed your best friend’s sweetheart, or you broke some kind of religious rule that’s making you feel guilty. Either way, the best way to get over feeling repentant is to repent, or acknowledge what you’ve done and promise not to do it any more.