I was paid in advance to do 10 sessions of tutoring, once a week. Unfortunately, I did not think to keep track of how many times it actually happened, as there were some weeks where it didn't work out. (I'm a teenager taking pretty much my first job. Since I noticed, I have been keeping track.) Looking back at a calendar, I estimate it was 10-12 sessions.
What should I do? I'm aware that I am at fault here for not keeping proper records. I think I should just apologize for my mistake, and treat all the ones that I didn't count as the ten that were paid for, even if it means I lose out a bit. Is this a good idea?