Of all the uncertain things in life, time is the constant that never falters. At least in our present state, time stops for no one. It rewinds for no one. The millions of decisions that we make every day are finite conclusions in the infinite vacuum of time. Don't let this intimidate you. No matter the choices you've made, you have more power than you think. Starting to put minor things into perspective in the grand scheme of things can have a profound impact on something that might seem permanent right now, in this moment. Will this really matter in, say, ten or twenty years? Even two months? When you're stuck with unproductive thoughts and feel like you're in a constant battle against yourself, that's your chance to lock that negative energy away and consider your problem from more than one angle.
Associating negative thoughts with the consequences of a choice you regret is like attaching yourself to a rock. Seriously, when you decide that every negative thing, every little frustration, is somehow tied to a decision that you blame all on yourself, it's like dragging a huge stone behind you every day. No matter how happy you may be in one moment, it's impossible not to wish that you could remove that rock from your back. It makes you slow and self-conscious, and keeps you from going very far. Think of negative energy as the tie that's keeping you attached to the rock that represents the choice you regret. Cut ties with negativity stemming from regret, and you can leave the rock behind and learn from your experience instead of lugging it around with you. Yay analogies!
Keep in mind - when dealing with regret, you don't have to force yourself to pretend that everything is okay. Sometimes we make choices that are flat-out bad. It's good to acknowledge that, and talking it out can be really beneficial. Once you've faced the negative aspects of your choice, take time to forgive yourself and move on.
If you regret something involving another person - a fight with a friend, a broken relationship that offered you no closure, an unrequited love you still hang on to - it's best to confront them in person if possible. Even if they aren't receptive to your attempts to build bridges and not burn them, I've personally found that making an effort and knowing that you have done everything in your power to promote healing and positivity can completely change your outlook on a situation. In an age where a one-word text might be the last contact two people ever have, face-to-face communication offers a more human solution.
Trust thyself. From now on, make choices that do two things: 1. reflect your values and priorities in a positive way, and simultaneously 2. challenge yourself to do the more daring, life-enhancing, positive thing. Staying strictly within one's comfort zone can lead to regrets of passivity, such as feeling like you've wasted your life or missed out on special moments. It's important to find that middle ground between sticking to your guns, staying safe and making choices that inspire you to try something new.