How can I switch my standard outlet to a GFCI?
Put the warning tape back on the LOAD terminals. 

Since you are sure the tabs on the old receptacle were not broken off, they were simply splicing the two wires (on that same side) to each other, as well as serving the receptacle. Use a wire nut to join them instead, then use a pigtail to serve only the LINE terminals of the GFCI.  

In this installation, LOAD will not be used.   

I have a feeling there are technical errors on the downline wiring.  Generally Code requires that currents be equal in any cable or conduit, and in a simple /2 Romex, there are only 2 wires so they must be equal, obviously. So who knows.  Much as I'd like to help you spend days tearing apart the whole room in the dark disassembling everything and ringing out every cable on a great bug hunt for a triangled neutral or bootlegged ground or old dimmer, that is not what you asked for today.