T-test analysis within one single subject

Hello,

I need to complete a single subject analysis comparing the mean outcome under two different conditions. However, the number of measurement is not the same.

For example:
The subject completed the test 15 times under the condition A, but 12 times under the condition B. I want to know if the outcome under conditions A and B differs.

Unpaired t-test is not correct, since it will assume that data between the two groups of observations are not uncorrelated, which is not the case since it is on the same subject.

Which statistical test should I use?

Thank you