My temp doesnt get higher than 150

i know theres a million threads on this and i have searched and read a lot of them and understand that the sensor is on the back of the head drivers side but i dont know if it would be the sensor since it starts at 100 then rises to 150 but then just sits there a floats around 145 to 155 never goes over 155 even on hot days driving for hours. i also know it could be the thermostat but i just put a new one in a couple of months ago and dont see how it could be stuck open. anyone know if the sensor could just be bad and somehow not letting the guage go past 155 or if a thermostat could really go bad so quickly. sorry for all the writing i was thinking to myself short and simple then this came out.