There is no problem in 3D printing I have lost more evenings to than bed levelling, and it is not even hard. It is just never finished. You level it, print three perfect things, and the next print's first layer comes out either smeared flat or floating above the glass laying down spaghetti.
The phrase "bed levelling" is half the trouble, because the bed is rarely the thing that moves. What moves is everything else: the nozzle height drifts as the gantry settles, the bed springs relax, thermal expansion changes the gap once the heater has been on for twenty minutes, and a stray blob of plastic stuck to the nozzle quietly adds a tenth of a millimetre nobody can see. You are not levelling a plane. You are chasing a gap that wants to wander.
My ritual now is dull and it works. Heat the bed and nozzle to printing temperature first, always, then level, because a cold bed lies to you. Clean the nozzle tip before touching anything. The classic sheet of paper under the nozzle, adjusted until there is a faint drag rather than free movement or a jam, gets me close enough every time. Then I print a single-layer test square in each corner and trust my eyes over the paper.
One day I will fit a probe and let the firmware mesh the bed automatically, and this whole ritual becomes a memory. Until then it is me, a sheet of paper, and a war of attrition I am content to keep narrowly winning.