A few of the issues are things like:
1. What do you take with you from the 21st century to establish your hermitage? Tools, fabric, storage containers, cooking utensils ... ? All these require skill and time to replicate which limits time available to deal with the day to day problems of putting food in the mouth. Many options are just too time expensive, skill levels inadequate unless learned and practiced beforehand and the location may not provide the required resources. Do you hoard prepper stye? Or scavenge by living on the edge of society - Possum and many others?
2. You need to adapt to the availability of resources (especially food) over the annual cycle. This usually requires agricultural skills plus storage and preservation techniques or migration.
3. Ever tried keeping fauna, possums especially out of a veggie patch without netting/wire/electric fence?