“unperfect actor on the stage, Who with his fear is put beside his part”
'Unperfect' means imperfectly prepared or not fully learned, not merely flawed. 'Put beside his part' means thrown off, made to forget his lines — 'beside' here carries the sense of 'away from' or 'outside of.' The actor fails not because he lacks talent but because stage-fright overwhelms what he knows.
“for fear of trust, forget to say The perfect ceremony of love's rite”
'Fear of trust' means fear of trusting himself — distrust of his own capacity to speak love adequately. 'The perfect ceremony of love's rite' means the right, complete form of declaration: the rite a lover is supposed to perform. The poet fails not because he feels less but because he mistrusts his own voice to carry the weight of what he feels.
“O'ercharg'd with burthen of mine own love's might”
'O'ercharg'd' means overloaded — carrying more than can be managed. 'Burthen' is an older spelling of 'burden.' The line sums up the octet's paradox: the poet's love is so strong that its very weight silences him, the way a cart overloaded with grain cannot move.
“let my looks be then the eloquence And dumb presagers of my speaking breast”
'Presagers' are heralds or advance messengers — things that announce what is to come. 'Dumb' means silent (not stupid). The poet asks his outward appearance ('looks') to serve as silent heralds of the love he cannot speak, his face and eyes declaring what his tongue withholds. 'Speaking breast' means a heart or inner feeling that is fully articulate even when the mouth is not.