Rapid treatment of mastitis is the best hope to save the ewe and udder. Prompt treatment will also help to reduce the reservoir of infection in the flock. Severe cases of mastitis should be euthanised without delay to avoid unecessary suffering.
The most common treatment is injectable antibiotic. Consult your vet to have a treatment plan for cases of mastitis. Prompt and early treatment increases the chances of recovery, so at least daily checking of ewes daily from lambing is advisable.
Giving anti-inflammatory or painkilling medication (from your vet) can increase the rate of survival and recovery by reducing inflammation and effects of toxins and also reduces pain.
Other suggested managements
There is potential value in stripping mastitic milk from the udder (into a container rather than directly onto the floor to avoid contamination) and treating with supportive oral fluid therapy, early on in acute cases to reduce the effects of dehydration.