Sorry, I know I am really stating the obvious - of course not being able to socialise and go out is going to have an adverse effect for many.
I know I am lucky - I am a single parent with my three teens so not alone (though they can all be dry and critical so...), but in my case the lockdown has really amplified feelings of loneliness, not being good enough, and general feelings of pointlessness and uselessness.
These are all there in a non lockdown situation but not having the usual distractions or opportunities to socialise has really brought them to the fore.