Transmission acts up over 45 mph

When I drive under 45 mph my car runs great with no issues, but when I go anywhere between 45 and 65 (second gear I assume)it acts like it wants to shut off. Or it struggles to get into the next gear. If I press the gas down and here the engine rev but my speedometer goes down. I turn the car off start it then all is well until I go over 45. I do not have a rpm gauge so I do not when it shifts. I changed the transmission filter so first gear works fine. I am told it might be a sensor on the transmission but I do not know.