A serpentine belt has an idler pulley on is to keep it tight at all times. If your serpentine belt is loose, then something is worn out. Its possible the idler pulley is stuck in its bracket so it cannot move or the spring on it is worn out.
If it is a serpentine belt the idler pulley more than likely needs replacing. If it is a drive belt you are adjusting it too tightly.
Pulleys misaligned? Defective tensioner?
Serpentine belts use an pulley to keep tension on the belt. You need a breaker bar; attach to the tension pulley and turn the nut. One way will not move and the other way will loosen the belt. It will usually then just slide off that pulley and then come off all the rest.
If it is a serpentine belt the idler pulley more than likely needs replacing. If it is a drive belt you are adjusting it too tightly.
Your belt is either to big or just plain worn out.
Check all pulleys for proper alignment. Check for a failed bearing in a pulley. Check for a locked up pulley.
The serpentine belt and the tensioner on a 300 SE motor is part of the fan system that controls all of the smaller motors on the front of the engine. The serpentine belt helps turn the alternator, power steering pump, water pump, and the fan. The tensioner is a pulley on this belt that helps to keep the belt tight enough to stay on, but not too tight to hinder operation.
Just to get it off? Try using a single edge razor blade. It will have what is called an idler pulley probably, most do, This pulley serves only one purpose, to keep the belt tight. This pulley can be levered, probably to a slack position, usually downward and the belt can then be removed.
Something is out of line, or you are using a warped belt. Take off and check the idler and also the tensioner pulley. They are most likely to cause belt problems.
Chrysler has a belt and pulley kit to fix the problem. it removes the smooth surfaced pulleys and replaces them with grooved pulleys. the new belt is grooved on both sides
The most common cause for a belt coming off is the belt tensioner is worn out. When this happens, the belt is not alighned with the other pulleys I.E. Power steering, A/C compressor, water pump.