The best of mindfulness is that even when you failed at being mindful and staying calm and such, there’s not judgement.
So, you failed and what? Let yourself to do it as an event that it’s gonna happen for sure along your practise. Use it as an alley. Get the information that is telling you, if need it.
Did you realised that you aren’t either letting yourself to feel frustrated, or angry. You are now frustrating yourself more for feeling frustrated. And so on. That happens with feelings and emotions. they are gonna be present in our lives, just as mind makes thoughts.

In Mindfulness failure doesn’t exist. It’s a continuos lifetime process to get information, knowing better situations or events, knowing the main actors in a story better to get a clearer mind of what’s happening in front of you and with you.

I know it, I’m being a know-all. This happens because I’m deep into mindfulness, relaxation for stress and anxiety management.

Congratulation because you realised and you are more aware now that you were before. I’m sure of that.