You could write about:
- missing your friends
- not being able to go out
- online schooling
- fear for older loved ones
- not doing sport/team activities
- being bored
It could go like
Dear diary,
Today was an awful day in the pandemic I missed my friends I usually see them at least once every week but lately I haven’t been able to see them and this is effecting me because I feel lonely, sad... etc etc