While it’s essentially the same as drawing a rectangle to draw on (as already suggested), you might try TIG’s “work plane” extension (if it works on your version) maybe it will fit/feel better for you? it will create a temporary surface to draw on to help constrain your work to that plane.
Also, the side panel isn’t square to the rest of the cabinet, if you turn on “axis by color” if that’s available in “styles” it will show those issues, this may be part of the problem you have too.
Also, (as another way to Box’s solution) if you don’t want to redraw what you have already done, you could select the single triangulated face (group it) lay it flat and use Eneroth’s “align to plane”, this will bring all geometry to one plane and you can then delete all the additional lines. Push pull that to your thickness and stand it up vertical again.
While these may work as "fixes’ to what you have, it would be better practice to try and draw clean from the get-go. Much like building real world items, it may be better for you to draw elements on the floor/a ground plane, to get your parts built and then stand them up and assemble? Sometimes when things are up in the air, they can be less manageable until you get the hang of it.