Sometimes inspiration is the hardest thing to have. I know that I've made mistakes, failed, and really felt it...multiple times. No one is exempt from this; it's an essential part of being alive.
When I do feel down in the dumpsters, I do allow myself to feel that way. I need to experience my emotions so that I can work through them. It's my way of processing fear, anger, and doubt and finding an empowering way out of it.
At the most troublesome times, that's when inspiration is the most important thing to have. When you're inspired, hold on to that feeling as much as possible. Embrace it. Save it. Use it to remind yourself of your purpose and goals every day.