“set me light”
'Set me light' means to hold someone in contempt, to regard them as worthless or of little account. The phrase is the opposite of 'making much of' someone. The poet imagines the youth deciding to treat him as negligible.
“prove thee virtuous, though thou art forsworn”
'Forsworn' means having broken a sworn oath — perjured. The poet says he will argue that the youth is virtuous even if the youth has betrayed the promises he made. The paradox is extreme: the advocate for the accused knows the accused is guilty.
“faults conceal'd, wherein I am attainted”
'Attainted' is a legal term meaning convicted of a crime, officially condemned, or tainted with disgrace. The poet claims he can produce a 'story' — an account, a legal narrative — of hidden faults for which he stands convicted in his own conscience. He will use his self-knowledge as evidence against himself.
“injuries that to myself I do, Doing thee vantage, double-vantage me”
'Vantage' means advantage or gain (from Old French 'avantage'). The poet argues that injuring himself for the youth's benefit ('doing thee vantage') rebounds as a double advantage to himself, because the act of loving the youth completely is its own reward. The punning repetition — vantage / double-vantage — drives home the paradox that self-harm and self-gain are the same act.