If the car is stop with the clutch pedal in and adjusted so there is a good air gap between the clutch disc and flywheel the cluster gear should be stopped, not spinning, so the reverse gear may not align well with the cluster gear making it harder to engage reverse, not make it grind. Putting in in any forward gear with the synchro working would stop the cluster gear from spinning if the clutch was not adjusted with enough air gap, correct
Good luck, don't give up, YOU can FIX it