Don't let it rattle around in your head. Dwelling on it serves no purpose other than to reinforce negative feelings your associating with your fluency. It's not easy letting go of these instances. Not letting it rattle around in your head is easier said than done (no pun intended.) It takes practice to learn to let go and move forward. You might start journaling these instances. Get it down on paper. Next time you find you're reliving it in your head, pull out what you wrote. Read it. Is there anything new on your mind? Add it. No new thoughts on the instance? Then why keep reliving it?