Solo levelling in New World can be extremely difficult. Some areas have an extremely high density of mobs, some with ability to dodge your attacks and deal massive amount of damage. This is especially true for solo healers.
I myself have reached level 60 as a solo healer recently and here are my top tips for levelling solo:
In New World, dealing with mobs with shields can be very problematic. They can block your normal light attacks using a shield with just a little bit of stamina. And their stamina will be back up very fast after depletion.
This is especially irritating for ranged weapons such as life staff and ice gauntlet, which do not have good single target spells.