How to lower the pressure in a pool?

I have a chlorine, below-ground pool. For the past month the pool has not been operating correctly. The pressure has been very high thus causing the entire pool to barely run, if at all. When I tried to lower the pressure of the pool, I preformed the normal steps. I applied DE to the skimmer and that did nothing and then I backwashed it. After I finished backwashing the pool it seemed to work perfectly, but the pressure steadily climbed, and after about 15 minutes the pressure was just as bad as before I did the backwashing. When I am backwashing or directly after, that circular thing (I don't know the name) that makes a vortex of water looks normal. But when the pressure skyrockets the vortex disappears and the water level drops inside of it. Another helpful fact that might be of help is this noise I hear. After I turn off the pool this high pitched noise comes from the giant pool filter. It dissipates after a few seconds and sounds like air rushing out of the pipes. Please help me, I will be forever in your debt.