I read this poem and I think it is about his struggle with faith as he never stops seeking an answer but he is also never able to find something satisfactory or good enough for him.
'everyone young going down the long slide
to happiness, '.
What is this long slide to happiness? Young is used here maybe not refer to people young of age but means people who are new to the faith. The young believers are maybe all 'going down' in search for 'happiness'. Instead of finding true happiness as the faith tells him he may have felt that it is a lie. He may have felt that it is a waste of someone’s mind to commit themselves to faith. He looks back at himself maybe wishing he was one of those young souls seeking God again, then finish with the thought that he would 'go down the slide' like 'free bloody birds'.
Larkin ends up, feeling lost and not being able to receive any direction.
It seems that the more Larkin seeked to find the Truth, the more delirious he got. I think that maybe that is Larkin's mood in his many attempts to find an answer.