Look at the Journey
What matters is the trajectory not the situation
You might be finding yourself in a bad day, where nothing goes like you planned It, where an inconvenient along the way makes everything go south or experience even personal tragedy. If you think you're the only one, don't worry, you're not alone. However, when a setback occurs the best way to look at It is to recognize as another pebble on the path of reaching your goals, not really paying too much attention to It or letting It affect your emotions in the long run, focusing in the overall journey towards your destination and feeling good because even If sometimes you don't get want you want momentarily, you know that you're only heading upwards. Let's begin by delving further more into this "Good Moments, Bad Moments" concept.
Mountains of joy and despair
A great problem that people experience is that when they look at the process of reaching any goal, learning any skill or just making something out of your life, they tend to confuse It as a lineal progress. This is not the case, everything you do in a prolonged fashion and in order to obtain knowledge/success is a complex system, which is composed of little parts and steps that are each as important for the functioning as a whole of your goal, so little deviations and malfunctions can occur, though with enough effort It will always go up again, making It take the shape of a sine wave. What I'm trying to say with this is that when you're trying to advance in something, you'll experience those bad days, where you don't see any progress or enjoyment, and good days, where everything seems to go great, with all the focus and motivation you can have. If you only look at what's happening to you right now you might get discouraged easily, though If you know where you're going and that It is just a part of getting there, you'll be much more resilient and able to perform.
I hope that with this little article I could have provided you with some information as to why grieving over a bad day can be a pointless thing, and how looking at the broader context and recognizing that you might falter along the way will make you keep working, knowing that no matter what, you're going forward. Thank you for your support and good luck!