Stuff happens. Let it. There's not much you can do about it. Dwelling on the past is a fool's game. Dwelling on the past is something that can hinder your future progress. Accept it, embrace it, move on, don't let it get to you. If you're constantly thinking about it, you're letting it get to you. Try to focus on being in the now. View your surroundings with intent. Feel the weight of your feet on the floor, your bum on the seat, your clothes on your back. Just enjoy and embrace being in the moment. Soon enough, you'll find that you've stopped letting things get to you.