Half my upstairs light ain't working. I've used a bulb that works to check. The 2 bedroom lights work but bathroom light hall and bath fan?

The flickering Bath Light is your clue to where the trouble is located. (Lights do not use GFCI, so forget about that) the problem will be found in the bath light fixture mounting box, or behind the light switch in the bath room. There is a loose connection in one of those bath room electrical boxes. With all the lights that are having trouble all turned on, uncover the bath room wall switch cover. Then remove the two screws that hold the switch in its box. Now that it is free, pull back on the two switch mounting ears, so as to wiggle the switch back out a little. If this activity has made any of the lights to flicker on/off you are on the spot. Look for loose side screw connections. Look for loose undersized white wire twist nut connectors. If it is not in the wall switch box, do the same in the bathroom ceiling light fixture box. Mostly i suspect a white wire bundle has come loose.

the lights and fan are probably powered off the wall outlet in the bath or nearby == turn off the breaker and inspect all the wall outlet sockets to be sure that you do NOT have any loose connections === if the power is a problem to you then best that you find the short or loose wire and prevent a home fire .......

They may be on the same circuit as your bathroom outlets, and those outlets should have GFCI protection. Check your GFCI plugs to make sure they are all working.