CRT Monitor and

The pot (variable resistor / preset on the circuit board inside the monitor) in the horizontal sync section needs cleaning, replacing & adjusting. but before that try the horizontal adj on the front panel of the monitor. Every monitor (digital or analogue) has it. Only if it doesn't clear the problem, try opening the monitor. Be careful so as not o venture too close to the EHT transformer.
Also make sure to degauze the CRT by earthing it (after switching it OFF) before toching any components inside. Also when you adjust the preset, make sure to use a highly insulated flat screw driver and wear heavily insulated rubber / PVC soled shoes because you'll have to adjust it with the monitor Powered ONonly then you'll be able to see the results. For your convenience, put a mirror in front of your monitor when you adjust the preset on the circuit board. Also make sure that you don't turn the preset very fast or put excess pressure on it as it may brea causing further damage to the circuit board.

Better still, if you don't wish to go deep into all this stuff, get it repaired by an authorised technician.