Possible Transmission issue?

The truck has to be shifted through gears to even move at all at first. It had plenty of transmission fluid surprisingly considering in 50,000 miles I hadn’t changed it. I can slow down and stop turn around and it seems like the truck loses momentum and doesn’t want to drive as well. I drove half a mile and it seemed like it was struggling and gradually slowing down. First 30MPH and then 20MPH. When I got home and pulled in from my little test drive. So, I concluded after staying stationary it loses the ability to move without being shifted through gears several times first and it gradually loses all its momentum while driving even short distances.