Could be a converter fill issue. What if you run it in neutral for 1 minute? Does it still feel like it slips? How does the fluid look? Level, color, smell? No same component (clutch or band) are applied in 1st or reverse. So the fault can not be attributed to a single clutch or the low/reverse band. The intermediate (kickdown/2nd) band is out of the equation at ii is applied only in 2nd gear.Ok on a cold start. It's the same in forward and reverse