It used to work fine on the middle setting but it only work on high and then freezes everything in the fridge too

The spring tension has changed in the thermostat. You will probably need a new one to correct the problem. You can get a universal replacement if you remove the old one and take it to an HVAC supply house. Ranco makes a good one for about $20.00. Make sure that when you replace the t-stat you have the length of capillary tube exposed that was exposed on the old one and it is in the same location. The excess cap tube must be coiled up carefully behind t-stat with NO SHARP BENDS or it will be ruined. Any sharp bend will stop it from working and you have to take your time to keep the cap tube from contacting any electrical parts. You will have to fit together the new t-stat with the supplied parts to make it like the old one.