97 corolla wont shift into last gear. And i have a new thermostat.

Hello, many times in this forum, we get questions like "this and this happened, and I just bought the car..." It's no good! I'm sorry to hear. If your engine speed is revving up without transmission engagement at highway speeds, it's almost a certain example of an internal transmission mechanical concern that will require repairs of significant cost. There's nothing for you to do other than seek professional guidance, unfortunately. https://repairpal.com/repair-shops

Push the OD button on shifter to enable overdrive.