It's important to self-motivation to not beat yourself up over these "bad days." For example, without these bad days, you have nothing to contrast with the "good days," so it's possible it makes you appreciate those times more. It can also give you insight into what produces a good or bad day for you, personally, if you start to monitor the circumstances around good and bad days in terms of how much sleep you get, your diet, general mood/feelings, etc.