When you pull the lever into "1", the valve body applies pressure to the Low/Reverse Band.
In other words, the BAND shares low and Reverse. If you get nor forward movement in "1", you have a band issue which is not uncommon.
This is different than when it is in D for Drive as it is going through a different circuit.
It sounds like you need a trans overhaul.
I don't know what your power level is and you may need some upgrades if it is higher than stock.
The 48RE can have (often does have) an issue with particles from band, clutch material and other debris.
For such a simple transmission, they can be quite a challenge to rebuild properly so it's necessary to have it rebuilt by someone who knows them well.