I have the new cartridge (Delta series 1300-1400) for my shower, I have removed the handle and trim

I am trying to replace the old cartridge, if only I could remove it. The instructions say to re-install handle on brass stem and rotate counterclockwise while lifting the cartridge out of the two notches on the sides of the body.
Not working as easily as said.
Is there a special tool, I don't want to break it or cause any more problems than I have now.

Please help.

I have the new cartridge (Delta series 1300-1400) for my shower, I have removed the handle and trim

I am trying to replace the old cartridge, if only I could remove it. The instructions say to re-install handle on brass stem and rotate counterclockwise while lifting the cartridge out of the two notches on the sides of the body.
Not working as easily as said.
Is there a special tool, I don't want to break it or cause any more problems than I have now.

Please help.
No special tool is needed. I see nothing holding it. Open up the image I put up and see for yourself. Perhaps a bit more muscle would help. Good luck, Tom

The tip is use penetrating oil and give it plenty of time to work, I put it on let set for a hour then repeated that, then like magic it comes right off.

I made the assumption you know that there is a brass ring that holds the cartridge in the housing.My previous post was to get that brass ring off, which it will not want to come off. Once the ring is off, then using pliers grab the cartridge and just work it back and forth and pull gently it will come out.