Need guidance for latching output of op amp (N-channel mosfet)
You can save yourself a lot of headache by using digital chips for digital functions. You can use an SR latch here, or a D flip-flop with a RESET input.
Also using a comparator with open-collector output in place of the op-amp allows you to eliminate the diode from your design. It also eliminates the need for a negative power supply (assuming the actual input voltage range is 0 - 2 V as shown in your schematic)
With an SR-latch it looks like this:



simulate this circuit &ndash; Schematic created using CircuitLab
Practically you might find it easier to source an \$\bar{S}\bar{R}\$ latch (i.e. with inverted inputs) --- then you'll have to swap around some connections to make things work.