Yep I feel that Times like those are when I recommend journaling. It's good to journal when you're feeling down, as it can help you process your emotions, but also it's good to write down the good times, so you can remember that there will be more fun days ahead. (Journaling is scientifically proven to be good for mental health, highly recommend)
That's kind of the eternal curse of bodybuilding/lifting tbh. When I'm at the gym, I have a good pump, and I look more muscular than normal because there's a lot of blood flowing to those muscles. But the other 23 hours in a day, I look small and weak by comparison. Half the reason I love working out is just for the pump, because it's the only time I feel like I look good xD