Delta 1700 Series Leaking (seat is stuck)

Hi,

My Delta shower head (T17251)has been leaking in between showers. I have no plumbing experience at all, but I watched about 100 videos and read articles and determined that I needed to check my cartridge and seats/springs. Everything looked okay to my untrained eye, but I spoke to Delta and they sent new seats and springs. I took the faucet apart and got everything done up until the part where I had to remove the seat. It seems like it's wedged into the pipe and not removable? Videos I watched either didn't have this white plastic piece OR they did but it had a screw on the bottom to remove it. I tried pulling it out with pliers but since it is plastic I didn't want to twist and break it,

Does anyone know if there's a way to get this piece out to replace it and the O-rings behind it or do I just replaced the springs and the eyeglass looking thing and leave the old seat?

Pictures attached of the current seat in the pipe and the handle (if that helps).

 

A pair of needle-nose pliers and a good firm tug should do it; it is held in by o-rings that did not let go when the rest of the body did.

 

Excellent, that worked. Now I can't get the new seat to push back far enough. I tried to just use the force of pushing the cartridge in and replacing the bonnut nut but the cartridge kept slipping out of place and when I tightened it the triangles on the cartridge were not staying in place.

 

Are you sure that you got the old seats out of the valve body? They would have been inside the two "holes" that the white piece was stuck in.