Time, I defy you: your monuments are merely recycled forms, and I refuse to believe that your innovations are novel; I will remain constant despite your corrosion.
Registers are official records; the term also suggests both written and musical registers, suggesting Time keeps all things in order.
The speaker addresses Time directly, dismissing its 'pyramids built up with newer might' as nothing novel or strange. They are mere 'dressings of a former sight'—recycled facades covering older structures. The speaker claims that because human lifespans are brief, we mistake recycled forms for genuine novelty and 'rather make them born to our desire, / Than think that we before have heard them told.' Time deceives through the brevity of human memory.
The sestet's defiance is total: the speaker defies both 'thy registers' (Time's records) and Time itself. Crucially, the speaker claims that 'thy records, and what we see doth lie'—that is, even the 'records' we believe to be objective fact are corrupted by Time's 'continual haste.' The only truth available is the speaker's own vow: 'I will be true despite thy scythe and thee.' Constancy becomes the one permanent form of truth.
Trends cycle: fashion, politics, technology all claim to be 'new' but are just old things repackaged. You're tired of the hype cycle, so you decide: I'm not going to chase novelty. I'll be steady and constant while everyone else chases the new thing.