Hard shifting and stalling

My pickup suddenly started shifting really hard at low speeds. A couple of days later, it started stalling at speeds over 40 mph. The engine idles roughly, the "check gauges" light turns on, and then the engine stalls. The "check gauges" light also comes on when I rev the engine above 3,500 rpm's while in Park. The check engine light is currently on, but that has been on since I bought the truck with 80,000 miles. It is related to trouble code P0453. I believe that trouble code has nothing to do with my problems. Right? So far, I've installed a new mass air flow sensor and air filter. I have also added a bottle of Lucas transmission fix and Lucas fuel injection cleaner. The transmission fluid and filter were changed last year. The levels are good and the fluid still looks red and clear. I've never had issues with this truck and all these issues were sudden. Your assistance will be greatly appreciated. Thank you.