Trying to increase resolution of ADC using an op-amp
This difference amp circuit should do the trick for you:

I have simulated the thermistor R4 to vary over time from 8k to 12k ohms.
The resistor R1, R3, and R5 were chosen to match the center of the thermistor range. This sets the common mode voltage to 2.5 volts, and amplifies the differential voltage caused by the thermistor varying with gain set by R2, I used 33k.

The blue trace is the resistance of the thermistor using the right axis. The black trace is the voltage going to the ADC.