Most people don't understand how he high side driver works.
C1,2 is a little battery that powers a amplifiers that drives the MOSFET's gate. (and unfortunately R3)
OUT must be at or near ground for a while so power can flow through D1 into C1,2 (charging them). The return path to ground is through OUT. (must be at ground for a while)
When MOSFET is on then C1,2 is sitting above 300 volts and there is nothing to charge C1,2. As long as there is a charge in your battery the MOSFET can be on. Long before the energy is discharged (into R3) the MOSFET must be turned off for a while so that you battery (C1,2) can be recharged.
I agree with all, R3 should not be 1k. Maybe 100k. Maybe no resistor.
It is common on the forum for some one to say "IR2110 not working" when they are trying to get the MOSFET to be on 100% of the time.