Finding an average with the lookup function

Hello, I am new to the forums and certainly not an expert at Excel. What I want to do is shown below:

=AVERAGE(LOOKUP(F56,F19:F55,C19:C55):C56)

Note that this function does not actually work. What I want to do is to take a value (F56), go and find where it is previously in the F column, return the result from the corresponding C column, and then average everything between that corresponding C column result and C56. This function does a good job at looking up the correct value, but I am unable to figure out how to do the averaging part. Any help would be greatly appreciated, as I have been stuck on this for hours. Thank you!