I can understand the situation you are in at the moment and I can get that it is getting difficult for you. I can see that it was not intentional that you skipped your practical class and you also know it wasn't that big a deal but kept thinking again and again about it, which made you feel worst. I can also see that you Know that it is ok to miss one class. This type of thinking is known as Rumination or worry which is associated with anxiety. You can try few things like thought stopping - Where you can simply clap your hands and say "STOP" loudly whenever you get this kind of thinking, this, of course, does not mean that you forget everything but will get you distracted and then you can slowly move on to Mindful breathing- Which is nothing but sitting in a comfortable position and focusing on your breath, try and calm your self down and inhale and exhale slowly, keep focusing on the way you breath. Try this for sometime and see if this helps you. It comes to my notice that you had visited a clinical psychologist earlier and that helped you so why don't you try and visit one or come online for a chat session with any of our experts.
The techniques mentioned above may only help you in distracting and taking away the thought from you for some time but it is recommended that you take regular sessions and try and work on coping with such thoughts.
Hope this helps you to some extent