1. Accept that things have changed 

The best way for me to get over something not going to plan, it just accept that the day isn't going to go as I had previously planned, and that's perfectly okay. Dwelling over the fact that something out of your control has happened meaning that you can't continue with your day as planned isn't going to help in the slightest. 

2. Clear your mind 

To move on to the next phase of your day, clearing your mind from whatever is clouding you up helps no end. Sometimes this could be going for a walk and getting fresh air, maybe having a little meditation time or sometimes it's as simple as just having a good cry. Afterwards I splash my face with water to freshen up and start the day again. 

3. Make a new plan 

This is often easier said than done, however when things aren't going to plan and you've come to terms with the fact that it's just one day and not the end of the world, make a new plan. Things may not be going to plan, but that's because the plan hasn't changed. For me, when things go wrong it tends to be due to mental health issues and so naturally the plan needs changing. If I can't get myself to leave the house because of my anxiety then I scrap the plan of going out and meeting friends or whatever, and focus on what I can get done at home. Laundry, cleaning up, organising my room are all productive things that I find I can manage when my anxiety doesn't allow me to stick to the previous plan.