repair the degauss circuit of your monitor which it seems is not functioning
(buy a replacement thermistor to start, as perhaps yours has gone bad)
if necessary check/replace the other components in the degauss circuit also. there wont be many to check
finally check the degauss coil itself (the wire surrounding your monitor screen edge) for damage/short circuit
alternative solution: buy a manual degauss wand/coil on ebay. however i have never used one so cant comment on effectiveness. i found some instructions though (for coil... no idea if method is same for a wand):
Hold the coil out in front of you and switch it on. Approach the middle of the picture tube as you swing the coil around in a small circle. Wipe it across the face of the tube in slowly widening circles. Take about five circles to get out to the outside perimeter of the screen and back away from the screen slowly as you continue to make big circles. When you have backed up a couple of feet and you are no longer effecting the picture, shut the coil off.