'It is plain that a life which includes deep resentment leads only to futility and unhappiness. To the precise extent that we permit these, do we squander the hours that might have been worthwhile.' (Page 66)
Note the 'these': it is in the plural. It is referring back not just to resentment but to the futility and unhappiness. If I feel my life is futile and I am unhappy, it is my fault. I am permitting these conditions because I am permitting resentment.