So the other day I drained and filled my tranny. When I drained it, only about 3 1/2 quarts came out. I didn't drain the cooler lines or try to get any out of the torque converter or anything. I didn't really want to drain it ALL anyway as I've heard it is not recommended when the fluid is burnt. So anyway, took 3 1/2 quarts out, put 3 quarts in plus about half a quart of Lucas tranny additive (not the stop leak stuff). I filled it until I got a steady dribble out of the check hole (yes I was on a solid FLAT surface and the truck was up to temp). Before the fluid change i was experiencing a slight late/hard shift intermittently. When I first changed it, it was shifting super smooth. But now almost a week later, the problem has worsened. Late/hard shifting at times and twice now it has refused to go into second gear. I've had to put it into neutral and then back to drive and it will shift. So my question is, am I doomed?? Or is there something I can do/try to fix it? Any advice is greatly appreciated as always.

Thanks!