Poor load regulation on SEPIC

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Hi,

I have build a SEPIC running at 500kHz switching frequency using the LT1370 chip. The prototype was built on a breadboard.(I understand that this is not ideal due to the parasitic capacitance and inductance)
Vin=9V
Vout=1.7V-12V

I don't quite understand why it has such poor load regulation at Vout=12V, when the load current is high. I have done the calculations for the components but it seems like the voltage drop is too significant.
What could be the reason? Or is it solely because it's running off a breadboard? Is it possible that my compensation design isn't correct?
Also, i don't fully understand how the current mode sensing works as shown in the block diagram, i read the operation in the datasheet but it is a bit vague.

Thanks in advance.

EDIT: I can't seem to add the datasheet and a picture of the prototype. Error keeps occurring stating that file is too large when clearly it's not. I apologize.
https://imgur.com/a/Ez9SOB7 I have uploaded them on imgur instead.