“Against my love shall be as I am now”
'Against' here means 'in preparation for' or 'by the time that' — not 'opposed to.' The poet is fortifying his verse against the future moment when his beloved will have aged as the poet already has. The word carries this anticipatory sense throughout the sonnet (see also line 13).
“Time's injurious hand crush'd and o'erworn”
Time is personified as a wrongdoer with a physical hand that crushes and wears out. 'Injurious' means 'causing harm' but also carries its legal sense of 'committing a legal wrong' — Time as a criminal who damages what he has no right to destroy. The image recurs across the sonnet sequence; compare Sonnet 65's 'sad mortality o'ersways their power.'
“his youthful morn Hath travell'd on to age's steepy night”
'Steepy' means 'steep' — a precipitous or abrupt drop. The youth's life is imagined as a single day: morning (youth) gives way to night (old age) not gradually but over a cliff edge. The word 'travell'd' adds a journey sense — life as a road that ends at the brink of a steep descent into darkness.
“do I now fortify Against confounding age's cruel knife”
'Confounding' means 'destroying utterly, ruining.' The word comes from Latin confundere, 'to pour together, to ruin,' and in Elizabethan English meant to overthrow or bring to nothing. Paired with the military verb 'fortify' and the weapon 'knife,' it turns age into a besieging enemy that the poet's verse must resist.